[SSSD] [PATCH] AD: Handle cases where no GPOs apply

[Stephen Gallagher]

It is possible to have a machine where none of the GPOs associated with
it include access-control rules. Currently, this results in a
denial-by-system-error.

We need to treat this case as allowing the user (see the test cases in
https://fedorahosted.org/sssd/wiki/DesignDocs/ActiveDirectoryGPOIntegra
tion

We also need to delete the result object from the cache to ensure that
offline operation will also grant access.

Resolves:
https://fedorahosted.org/sssd/ticket/2691