Possibility of modifying NSG SecurityRule priorities, fix for #990

[Thorium]

This PR closes #990

The changes in this PR are as follows:

• Changed to NSG Builder only, a new keyword priority_incr can be used to replace previously hard-coded value.
• For max backward combability, the default value is the old hard-coded builder value, 100.

I have read the contributing guidelines and have completed the following:

• [x] Tested my code end-to-end against a live Azure subscription. See below...
• [x] Updated the documentation in the docs folder for the affected changes.
• [x] Written unit tests against the modified code that I have made.
• [x] Updated the release notes with a new entry for this PR.
• [x] Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here: I did test that the values are correct, but this was a builder-side hard-coded value parametrization only, so there is nothing to be added to the data-models or ARM templates, so no changes to deploy to Azure.

[Thorium]

This is a very tiny changeset. The CI failure is definitely not my fault, it fails on dotnet configs and I've not changed anything there.

[ninjarobot]

Thank you for the fix...that's an odd limitation I wasn't aware of. I appreciate you made this backwards compatible.

The CI failure is for the fantomas check and can be fixed by running dotnet fantomas src -r (local tool installed automatically on restore) and committing the updates in your PR.