sysdb: add sysdb_domain_set_enabled()
    
    This will be used in subsequent patches to disable subdomains.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/4009
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    (cherry picked from commit 7a03e99890806257df1ed8a126673d6a032fee6a)