KCM: Fall back to using the first ccache if the default does not exist
Resolves:
https://pagure.io/SSSD/sssd/issue/3838
KCM stores the default ccache in a separate DB entry. If the DB entry
contains a UUID that cannot be found in the DB for whatever reason, we
should just use the first ccache as the default. (This is what we
already do if there is no default)
Reviewed-by: Pavel Březina <pbrezina@redhat.com>