Replace ipalib with ipa command line
    
    Ipsilion uses ipalib just to parse FreeIPA's config file and to create a
    service principal. This can be easily archived by shelling out to the
    ipa command line tool. The server, realm, and domain names are not
    required either.
    
    * Use subprocess.check_output()
    * ipa-getkeytab no longer needs '-s server' argument. It can parse
      /etc/ipa/default.conf on its own.
    * The service principal name doesn't need the @REALM suffix. FreeIPA
      adds it automatically.
    
    The change is necessary, because FreeIPA 4.8 will drop support for
    Python 2 and Ipsilon is still not fully ported to Python 3.
    
    See: https://fedoraproject.org/wiki/Changes/FreeIPA_Python_2_Removal
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>