tests: change the Azure VM size on CI to B2s

[renanrodrigo]

Why is this needed?

This PR solves all of our problems because the A2_v2 VMs we have been launching is not compatible with the latest daily noble images provided by pycloudlib.

Discarded Ideas:

• Changing the daily image on pycloudlib to gen-1: could affect other users of the lib for no good reason
• Passing the image string ourselves instead of querying pycloudlib: a different flow in our code for no good reason. And gen-2 should be the default from now on.
• Opening an issue against pycloudlib to ask for images of different generations: could take potentially a lot of time there, and would still be a different code flow for us for no plausible reason.

Fixes: #3121

Test Steps

From a branch with this patch: tox -e behave -- -D releases=noble -D machine_types=azure.generic Not sure if it passes but all machines get created.

Also, testing other images for other releases shows they are all supported on the new instance type.

Checklist

• [ ] I have updated or added any unit tests accordingly
• [x] I have updated or added any integration tests accordingly
• [ ] Changes here need to be documented, and this was done in:

Does this PR require extra reviews?

• [x] Yes - @paride could see it even after merged for an opinion :D
• [ ] No

[github-actions[bot]]

Jira: This PR is not related to a Jira item. (The PR title does not include a SC-#### reference)

GitHub Issues:

• Fixes: #3121

Launchpad Bugs: No Launchpad bugs are fixed by this PR. (No commits have LP: #### references)

Documentation: The changes in this PR do not require documentation changes.

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