[RPD-319] Fix multiple subscription group bug

[Christopher-Norman]

This PR fixes a bug which would cause Matcha to fail if the user has more than 1 subscription group.

We now get the active set subscription group from the az CLI tool.

Checklist

Please ensure you have done the following:

• I have read the CONTRIBUTING guide.
• I have updated the documentation if required.
• I have added tests which cover my changes.

Type of change

Tick all those that apply:

• Bug Fix (non-breaking change, fixing an issue)
• New feature (non-breaking change to add functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (add details above)