xenial queens dvr install removes neutron-l3-agent

[Edward Hope-Morley]

Just deployed a Xenial Qeens from the next charms with DVR enabled and noticed that the neutron-l3-agent package had been purged by the charm thus rendering my deployment broken. There was some code recently landed [1] to enable dvr-snat that appears to be the culprit specifically:

        if (use_l3ha() and
           CompareOpenStackReleases(_os_release) >= 'newton'):
            install_l3ha_packages()
        else:
            packages_to_purge.extend(L3HA_PACKAGES)

L3HA_PACKAGES is keepalived but since that package is installed as a dependency of neutron-l3-agent, purging it causes the latter to also get removed.

https://github.com/openstack/charm-neutron-openvswitch/commit/1486c83

[Edward Hope-Morley (hopem)]

Patch: https://review.openstack.org/#/c/642532/