Add ability to create usergroup based policy rules

[samveen]

The PR is to implement feature #1848

##Feature description##
In the case of lots of users (or user churn) of a cluster, it is easier to add a group based policy instead of creating individual policies for users.

Code logic:

• Given the request's peername, get the groups for the user.
• If the group list contains the rule name, it's a match, so carry forward as previously, or check for username and carry forward as previously.

The UT result

As I no longer have access to an xCAT environment, I am unable to unit-test this.

[samveen]

@gurevichmark @whowutwut would you please review this?