RFE: ipa client should setup openldap for GSSAPI
    
    The IPA client installer currently edits /etc/openldap/ldap.conf, setting up
    the client to consume LDAP data from IPA.  It currently sets:
    URI
    BASE
    TLS_CACERT
    
    This PR makes ipa-client to add this AV pair:
    SASL_MECH GSSAPI
    
    Resolves: https://pagure.io/freeipa/issue/7366
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>