AzureDevOpsDsc: Fix style so it follows style guideline

[johlju]

We should look at the style guideline and compare it to the code to see what areas need to accommodate for PowerShell classes, and also modify the code towards the style guideline.
We should do this before we get to many resources that diverge to much from the style guideline.

https://dsccommunity.org/styleguidelines/

We could use this issue to list the suggested additions to the style guideline, or accommodations. I can help make the appropriate changes to the style guideline.

[johlju]

Add to style guideline: Properties that are Key should not also be set to Mandatory as Key is implicitly mandatory.