Install hook fails when principal charm is py3

[Chris MacNaughton]

When the principal charm is py2, apt_pkg gets installed and is available for ceilometer-agent; however, when the principal is py3, cielometer-agent fails its install hook

2017-11-24 07:40:53 DEBUG install Traceback (most recent call last):
2017-11-24 07:40:53 DEBUG install File "/var/lib/juju/agents/unit-ceilometer-agent-2/charm/hooks/install", line 52, in <module>
2017-11-24 07:40:53 DEBUG install CONFIGS = register_configs()
2017-11-24 07:40:53 DEBUG install File "/var/lib/juju/agents/unit-ceilometer-agent-2/charm/hooks/ceilometer_utils.py", line 95, in register_configs
2017-11-24 07:40:53 DEBUG install release = get_os_codename_package('ceilometer-common', fatal=False) \
2017-11-24 07:40:53 DEBUG install File "/var/lib/juju/agents/unit-ceilometer-agent-2/charm/hooks/charmhelpers/contrib/openstack/utils.py", line 429, in get_os_codename_package
2017-11-24 07:40:53 DEBUG install import apt_pkg as apt
2017-11-24 07:40:53 DEBUG install ImportError: No module named apt_pkg

[James Page (james-page)]

commit d73568eb13e1f696a447ef72d2cbcd3e3bbb90ec
Author: Liam Young <email address hidden>
Date: Fri Nov 24 09:21:44 2017 +0000

    Install python-apt before hooks run

    Previously this charm had relied on python-apt being installed by
    the principle. As charms migrate to py3 this is no longer a safe
    assumption and actually never was. This change ensures that
    python-apt is installed before immediatly. This follows the same
    pattern as other charms by adding an install and install.real
    files.

    Change-Id: I9fe2a031df736b867d4f277448a4d8649fa8de64