Use self.opt_in_group overrides

Use of self.opt_in_group makes option setting "sticky" even when
subsequent configuration files are loaded in. In some cases
(e.g. ``test_cert_setup.CertSetupTestCase``) the directly set
override (not using ``opt_in_group``) could be lost as test further
test cleanup patchsets were proposed. All identified direct
CONF object option setting has been replaced with use of
``opt_in_group``.

Change-Id: Iccd72003b61f775b71ea7402dc70eba1723d2336
bp: keystone-parallel-testing