SSSDConfigTest: Try load saved config
    
    Python module SSSDConfig should be able to save configuration file
    and later load the same configuration file without problem.
    
    Unit test for:
    https://fedorahosted.org/sssd/ticket/2837
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    (cherry picked from commit 87ef67286b64af98d32a3a5abcd28a9c2886f751)
    (cherry picked from commit 69612bc5d0a9219ecccf3e8c6410059322aeecc6)