Add config to install MAAS Release Version

[MaikRe]

Why/What?

Adds an optional MAAS_RELEASE_VERSION to config.sh which can be set to any release candidate, i.e. 3.5/stable.
Assuming an existing dev environment, this allows a user to simply do:

> lxc delete maas-dev --force
> /setup-dev-env.sh -su -sd -si -sn -sc -ss --ok

This is the alternative to manually having to do

> lxc launch ubuntu:24.04 other-maas-dev-container -p default -p maas-dev
> # all the other dhcp, user account, setup here

This allows quick comparison of MAAS behavior on existing releases.

Note

• tested only for 3.5 and 3.6

[skatsaounis]

LGTM +1

[tmerten]

@MaikRe Was it on purpose not to refresh the snap tree with a release version?

[tmerten]

As discussed in person I am somewhat reluctant to merge this as is. I think this is mixing two different use cases.

1. setting up a maas dev environment with the ability to change and test code
2. setting up a maas prod-like environment to test things (but lacks the ability to create a patch/fix for this particular branch/version easily)

IMO we should solve this more as in:

1. check out the correct branch related to the version under test
2. spin up another network/vm/project with this branch

This way we can offer the same experience we offer for development on edge for a particular version.

Unfortunately the solution idea sketched above is a much harder task. That's mostly because networks need to be unique in LXD and cannot be tied to a project. So the script would somehow need to associate networks with MAAS versions or keep state of the networks already created to make sure that each installation/project would get its own network(s).

[tmerten]

see comment above