ipa-getkeytab: add option to discover servers using DNS SRV
    
    The basic flow is:
    
    - If server is provided by the user then use it
    - If server the magic value '_srv', check for _ldap._tcp SRV records for
      the domain in /etc/ipa/default.conf
    - If no servers are found use the server from default.conf
    
    https://pagure.io/freeipa/issue/8478
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>