Bionic not available for commissioning on pro-enabled systems

[Marcus Boden]

Maas does not support using Bionic image for commissioning, even though the systems are pro-enabled and therefore still have support for Bionic.

The problem is that we're running an airgapped system and cannot simply sync all necessary focal packages for commissioning (even if the deploy is bionic again). This causes commissioning to fail due to the missing packages.

Maas uses the distro-info package to check get the supported releases by calling the `supported()` function [1]. Changing this to `supported_esm()` fixes the issue.

To fix that on a running and snapped system, you can use an overlay mount like this:
```
cp /snap/maas/current/lib/python3.8/site-packages/provisioningserver/drivers/osystem/ubuntu.py .
# change line 57 to use supported_esm().
vim ubuntu.py
mount -o ro,bind ubuntu.py /snap/maas/current/lib/python3.8/site-packages/provisioningserver/drivers/osystem/ubuntu.py
```
Above is a rather dirty hack, so use at your own risk.

[1] https://git.launchpad.net/maas/tree/src/provisioningserver/drivers/osystem/ubuntu.py?h=2.9#n57

[Jerzy Husakowski (jhusakowski)]

MAAS should always look at the supported-esm list.