[SSSD] [PATCH] SSSDConfig: Fix nonfunctional SSSDDomain.remove_provider()

[Stephen Gallagher]

We were removing the provider from the dictionary of enabled providers,
but we were not deleting the actual *_provider key/value pair from the
option dictionary. So we'd remove all options related to this provider
from the configuration, but we wouldn't actually remove the provider
itself (when the configuration was written back out).

Also adds a regression test to the unit test suite.

https://fedorahosted.org/sssd/ticket/1388