Drop testing with PostgreSQL

neutron completely removed its code to support PostgreSQL by [1].
Switch the testing coverage to use mysql instead, even in stable
branches, because any problems are not likely to be fixed.

[1] https://review.opendev.org/c/openstack/neutron/+/934171

Change-Id: I9bde4b20212d6111a75d91059f8e983d86ba9b59
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>
2 files changed