Tempest can be aware of pre-provisioned networks

Some tests are against preprovisioned networks if static accounts are used,
e.g.
https://github.com/openstack/tempest/blob/7e96c8e854386f43604ad098a6ec7606ee676145/tempest/api/compute/admin/test_auto_allocate_network.py#L79
So patch makes that tempest takes
projects without networks if tests do not require networks.
If tests require networks then tempest takes projects
with pre-existing networks.
To swithch on the new capability set config option
[auth]separate_projects_by_network_existence to true.

Related-PROD: https://mirantis.jira.com/browse/PRODX-20816
Change-Id: I77dc96ad9f30f3a258dea5b098d3c8090dc2a814
7 files changed