Avoid joins in _server_group_count_members_by_user
Avoid joins in _server_group_count_members_by_user

When pulling instances out of the cell databases we
only care about counts, so there is no need for the default
joins on security_groups and instance_info_caches tables from
InstanceList.get_by_filters. The default joins happen in
instance_get_all_by_filters_sort in the DB API.

Change-Id: I04cff5f9b4bcca1b1d143b82d7490cbc4d88ebe4

mikalstill/nova,gooddata/openstack-nova,mikalstill/nova,rahulunair/nova,klmitch/nova,mahak/nova,mikalstill/nova,gooddata/openstack-nova,klmitch/nova,klmitch/nova,openstack/nova,klmitch/nova,gooddata/openstack-nova,rahulunair/nova,mahak/nova,openstack/nova,mahak/nova,rahulunair/nova,gooddata/openstack-nova,openstack/nova