[stable/antelope] Add class for keystone audit middleware testing

[MylesJP]

Added general class for testing keystone audit middleware functionality in charms. Tests correct rendering of api-paste.ini file and allows charms without an api-paste.ini file to skip the tests.

Example usage with charm Heat:

tests/tests.yaml

tests:
  - zaza.openstack.charm_tests.audit.tests.KeystoneAuditMiddlewareTest

tests_options:
  audit-middleware:
    service: heat

(cherry picked from commit ff5cdf51e6ab7536993ba4ebd15bdfaaa180122e)