Added tests for checking expand/contract branch upgrade
Added tests for checking expand/contract branch upgrade

Currently tests check only that "upgrade head" perfoms without
errors. This change adds tests that will check that
upgrade --expand/contract also working without errors for both
MySQL and PostgreSQL.

Also move test_branches and test_has_offline_migrations* to
_TestModelsMigrations, so all subprojects will have this
tests running for there directories as well.

Closes-bug: #1613318

Change-Id: I3bff046ed83dd8ed21da20ab9e4331e95047e2b6

eayunstack/neutron,igor-toga/local-snat,openstack/neutron,noironetworks/neutron,mahak/neutron,huntxu/neutron,noironetworks/neutron,eayunstack/neutron,openstack/neutron,cloudbase/neutron,huntxu/neutron,igor-toga/local-snat,openstack/neutron,sebrandon1/neutron,mahak/neutron,mahak/neutron,cloudbase/neutron,sebrandon1/neutron