SYSDB: Index the ccacheFile attribute
Related:
https://pagure.io/SSSD/sssd/issue/3968
The Kerberos ticket renewal code searches for user entries which have
the ccacheFile attribute set. Since the search can potentially traverse
all the users, it might be a good idea to index the attribute.
Reviewed-by: Alexey Tikhonov <atikhono@redhat.com>