os_release() uses default release in install hook

[Corey Bryant]

os_release determines the OpenStack release in this order:
1) based on an installed package
2) based on openstack-origin
3) default

def os_release(package, base='essex', reset_cache=False):
    ...
    _os_rel = (
        get_os_codename_package(package, fatal=False) or
        get_os_codename_install_source(config('openstack-origin')) or
        base)
    return _os_rel

Since neutron-openvswitch is a subordinate charm, it doesn't have an openstack-origin config option. Therefore, when called via determine_packages() in the install hook:

    cmp_release = CompareOpenStackReleases(
        os_release('neutron-common', base='icehouse'))

the order results in:
1) None (because 'neutron-common' isn't yet installed)
2) None (because neutron-openvswitch doesn't have openstack-origin)
3) icehouse (because it's the default/base)

This causes the charm to attempt to install the old neutron-plugin-openvswitch-agent package, which was removed in rocky. See determine_packages():

    if cmp_release >= 'mitaka' and 'neutron-plugin-openvswitch-agent' in pkgs:
        pkgs.remove('neutron-plugin-openvswitch-agent')
        pkgs.append('neutron-openvswitch-agent')

[OpenStack Infra (hudson-openstack)]

Change abandoned by Corey Bryant (<email address hidden>) on branch: master
Review: https://review.openstack.org/594516
Reason: There's already a fix in progress for this at: https://review.openstack.org/#/c/592343/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/592343
Committed: https://git.openstack.org/cgit/openstack/charm-neutron-openvswitch/commit/?id=56d60366c775b309c6d989d4ac0399a3ef10e17e
Submitter: Zuul
Branch: master

commit 56d60366c775b309c6d989d4ac0399a3ef10e17e
Author: James Page <email address hidden>
Date: Thu Aug 16 11:36:46 2018 +0100

    Deal with lack of neutron-plugin-openvswitch-agent

    The neutron-plugin-openvswitch-agent package has been dropped @ Rocky.

    Pre-install neutron-common and ensure any previously cached os_release
    value is reset when evaluating which OpenStack release is being installed,
    ensuring that the correct package for the neutron-openvswitch-agent is
    used.

    Closes-Bug: #1788266
    Change-Id: I1224aa7f5e7caa6a8aaf2ab3043fac9c62735749