Fix get_request_group_mapping doc
Fix get_request_group_mapping doc

The RequestSpec.get_request_group_mapping method's doc wrongly states
that that return value is a dict of UUID values. Actually it is a list
of UUIDs.

Change-Id: I75b02757b319ea0f706a9c4167a227f5a043c37a

openstack/nova,klmitch/nova,klmitch/nova,rahulunair/nova,mahak/nova,openstack/nova,mahak/nova,openstack/nova,mahak/nova,klmitch/nova,rahulunair/nova,klmitch/nova,rahulunair/nova