Configuration parsing updates
    
    These changes are all related to following ticket:
    https://fedorahosted.org/sssd/ticket/763
    
    Changes in SSSDConfig.py merge old and new domain record instead of just
    deleting the old and inserting the new one. The old approach let to loss
    of some information like comments and blank lines in the config file.
    
    Changes in API config were performed so our Python scripts (like
    sss_obfuscate) don't add extra config options to the config file.