Rename CustodiaModes.STANDALONE to CustodiaModes.FIRST_MASTER
This is related to the DL0 code removal. FIRST_MASTER describes this
mode a lot better.
See: https://pagure.io/freeipa/issue/7689
Signed-off-by: Thomas Woerner <twoerner@redhat.com>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>