Network securty groups option to set initial rule priority

[ninjarobot]

The changes in this PR are as follows:

• Network Security Groups: Added option to set priority for initial SecurityRule.

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

• [x] Tested my code end-to-end against a live Azure subscription.
• [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:

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

    nsg {
        name "my-nsg"
        initial_rule_priority 1000
        priority_incr 50
    }

[ninjarobot]

@UrviGoel can you please review? Will this solve your need for now? It will take a bit longer to add support for linking security rules to an existing NSG as requested in #992.

[UrviGoel]

Hey, this should work for me. Thank you!