Add a new GroupAffinityFilter

1) Add a new filter to schedule the instance on to host from a set
   of group hosts
2) Update filter_scheduler.rst to add the new GroupAffinityFilter
   filter description

Implements blueprint group-affinity-filter

DocImpact

Change-Id: Iba59764e692cd1d3bc80521ba5a4412f2bebb168
Merge "Add a new GroupAffinityFilter"