Nova: Limit stable/newton to Trusty based grenade jobs
Nova: Limit stable/newton to Trusty based grenade jobs

This is required as stable/mitaka devstack does not support Xenial and
will cause the grenade job to fail during the initial install.

Change-Id: I06e7d65256c6a82854b733142cdb961ebe1525f9
Closes-bug: #1642543

Tesora/tesora-project-config,openstack-infra/project-config,Tesora/tesora-project-config,openstack-infra/project-config