Add excluded_members support to the rest of the group providers.

Since Usermod doesn't support removing a user from a group, I've added necessary error messages that reflect :excluded_members is not supported with it. And since AIX is a subclass of Usermod provider it will not be supporting this new option as well. Note that it also doesn't support append property. 

thoughts @danielsdeleo / @lamont-granquist ?

/cc: @adamedx FYI about AIX group provider