How to use the charmhelpers.fetch.apt_install function in charmhelpers

def install_hook():
    # Run both during initial install and during upgrade-charm.
    if not os.path.exists(default_haproxy_service_config_dir):
        os.mkdir(default_haproxy_service_config_dir, 0o600)

    config_data = config_get()
    source = config_data.get('source')
    if source == 'backports':
        release = lsb_release()['DISTRIB_CODENAME']
        source = apt_backports_template % {'release': release}
        add_backports_preferences(release)
    add_source(source, config_data.get('key'))
    apt_update(fatal=True)
    apt_install(['haproxy', 'python-jinja2'], fatal=True)
    # Install pyasn1 library and modules for inspecting SSL certificates
    apt_install(['python-pyasn1', 'python-pyasn1-modules'], fatal=False)
    ensure_package_status(service_affecting_packages,
                          config_data['package_status'])
    enable_haproxy()

group='nogroup')
        # Touch
        with open(jenkins_bootstrap_flag, 'w'):
            pass

    log("Stopping jenkins for plugin update(s)", level=DEBUG)
    service_stop('jenkins')
    install_jenkins_plugins(jenkins_uid, jenkins_gid)
    log("Starting jenkins to pickup configuration changes", level=DEBUG)
    service_start('jenkins')

    apt_install(['python-jenkins'], fatal=True)
    tools = config('tools')
    if tools:
        log("Installing tools.", level=DEBUG)
        apt_install(tools.split(), fatal=True)

def install():
    """Basic Ceph client installation."""
    ceph_dir = "/etc/ceph"
    if not os.path.exists(ceph_dir):
        os.mkdir(ceph_dir)

    apt_install('ceph-common', fatal=True)

def update_nrpe_config():
    # python-dbus is used by check_upstart_job
    apt_install('python-dbus')
    hostname = nrpe.get_nagios_hostname()
    current_unit = nrpe.get_nagios_unit_name()
    nrpe_setup = nrpe.NRPE(hostname=hostname)
    nrpe.add_init_service_checks(nrpe_setup, services(), current_unit)
    nrpe_setup.write()

def _ensure_packages(self, packages):
        '''Install but do not upgrade required packages'''
        required = filter_installed_packages(packages)
        if required:
            apt_install(required, fatal=True)

def __init__(self, endpoint, **kwargs):
        try:
            from keystoneclient.v3 import client
            from keystoneclient.auth import token_endpoint
            from keystoneclient import session
            from keystoneclient.auth.identity import v3
        except ImportError:
            if six.PY2:
                apt_install(["python-keystoneclient"], fatal=True)
            else:
                apt_install(["python3-keystoneclient"], fatal=True)

            from keystoneclient.v3 import client
            from keystoneclient.auth import token_endpoint
            from keystoneclient import session
            from keystoneclient.auth.identity import v3

        self.api_version = 3

        token = kwargs.get("token", None)
        if token:
            auth = token_endpoint.Token(endpoint=endpoint,
                                        token=token)
            sess = session.Session(auth=auth)
        else:

def ensure_packages(packages):
    """Install but do not upgrade required plugin packages."""
    required = filter_installed_packages(packages)
    if required:
        apt_install(required, fatal=True)

def ns_query(address):
    try:
        import dns.resolver
    except ImportError:
        apt_install('python-dnspython')
        import dns.resolver

    if isinstance(address, dns.name.Name):
        rtype = 'PTR'
    elif isinstance(address, six.string_types):
        rtype = 'A'
    else:
        return None

    answers = dns.resolver.query(address, rtype)
    if answers:
        return str(answers[0])
    return None

def install_packages():
    """
    Install required packages to build the k8s source, and syndicate between
    minion nodes. In addition, fetch pip to handle python dependencies
    """
    hookenv.log('Installing Debian packages')
    # Create the list of packages to install.
    apt_packages = ['apache2-utils',
                    'build-essential',
                    'git',
                    'make',
                    'nginx',
                    'python-pip', ]
    fetch.apt_install(fetch.filter_installed_packages(apt_packages))

def ensure_packages(packages):
    """Install but do not upgrade required plugin packages."""
    required = filter_installed_packages(packages)
    if required:
        apt_install(required, fatal=True)