Add notes for network and identity in get_service_list
Add notes for network and identity in get_service_list

This commit adds notes for network and identity service in
get_service_list(). Sometimes people (including me) are confusing that
only the two services are `True`. So, this commit is trying to clarify
it.

Change-Id: I0465cb6de2b2f0cfdfee0948e2c3ebc0133357ae

Juniper/tempest,vedujoshi/tempest,Juniper/tempest,cisco-openstack/tempest,openstack/tempest,cisco-openstack/tempest,masayukig/tempest,vedujoshi/tempest,masayukig/tempest,openstack/tempest