Add test to verify FQDN hostname sanitization

This scenario has impacted field engineers trying to spin up
instances with names containing freeform characters alongside
openstack-designate service for neutron.

Also, adding a config feature flag to enable/disable the test.

The get_class_validation_resources wrapper can only manage
one instance of class level validation resources that is used
by all tests defined in the parent class. However, this testcase
requires a custom server name of FQDN type be defined and managed
for the test to be verified successfully. Hence, placing it in a
separate class to prevent base class tests from failing.

Relates to https://bugs.launchpad.net/nova/+bug/1581977 and
gerrit https://review.opendev.org/c/openstack/nova/+/764482/

Devstack gate test:
[Master] https://review.opendev.org/c/openstack/devstack/+/801366

Change-Id: I6a89824e9b2d1b2387e35e55056ad251df3e8633
2 files changed