Unable to locate package python3-octaviaclient (and python-octaviaclient)

[Alvaro Uria]

The charm uses layer-basic to install APT packages that only exist from bionic onwards.

* When charm-openstack-service-checks is deployed in xenial, an error will happen (see [1])
* python-octaviaclient package availability in distro mirrors [2]
* python3-octaviaclient package availability in distro mirrors [3]
* in xenial-queens ubuntu-cloud archive, both packages exist [4]

We need to verify on the charm code if:
1) xenial-queens is used
2) bionic is used (both packages)
3) any later distro series is used (only python3-octaviaclient would be installed)

1. https://pastebin.ubuntu.com/p/ZnJGHcPKSd/
2. https://packages.ubuntu.com/search?keywords=python-octaviaclient&searchon=names&suite=all&section=all
3. https://packages.ubuntu.com/search?keywords=python3-octaviaclient&searchon=names&suite=all&section=all
4. http://ubuntu-cloud.archive.canonical.com/ubuntu/dists/xenial-updates/