MAAS Latest stable (ppa:maas/stable)

[SysXpert]

Ubuntu: focal

When attempting to install the charm without passing any `maas_source` option it will fail.
It seems that `maas_source` should be made a `Required` option or at least have as a default ppa the latest version of maas i.e. ppa:maas/2.9
The stable ppa has being deprecated with result to break the `apt-get update` with the following error:
```
apt-get update
Hit:1 http://ubuntu-cloud.archive.canonical.com/ubuntu focal-updates/victoria InRelease
Get:2 http://security.ubuntu.com/ubuntu focal-security InRelease [109 kB]
Hit:3 http://archive.ubuntu.com/ubuntu focal InRelease
Ign:4 http://ppa.launchpad.net/maas/stable/ubuntu focal InRelease
Get:5 http://archive.ubuntu.com/ubuntu focal-updates InRelease [114 kB]
Err:6 http://ppa.launchpad.net/maas/stable/ubuntu focal Release
  404 Not Found [IP: 91.189.95.85 80]
Get:7 http://archive.ubuntu.com/ubuntu focal-backports InRelease [101 kB]
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/maas/stable/ubuntu focal Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
```

[Alex Kavanagh (ajkavanagh)]

Note: affects deployments with ocf:maas resources on Focal and later (which is, obviously, now, a more frequent target).

Workaround (as implied in the report by the OP) is to set the maas_source option to "stable/<version>" in the deployment bundle to match the version of MaaS that it is being deployed to.

Fix is to (on focal+) check whether it is default, and block if it is not set and an osf:maas resource is detected. Relevant search in 'hook.py' is to search for 'ocf:maas' and trace the called functions.