rgw/kms/vault - document configuration for new transit logic

rgw/kms/vault - document configuration for new transit logic

Using the new transit logic requires slightly different configuration.
additionally there is some backwards compatibility support, which
also needed documentation.

The existing description of how to configure hashicorp vault
to work with ceph was also incomplete.  I've fleshed that out a bit,
including considerably more information on how to use configure
and use the vault secret agent with ceph.

Fixes: http://tracker.ceph.com/issues/48746
Signed-off-by: Marcus Watts <mwatts@redhat.com>
(cherry picked from commit a9edb632a5a8a768b1cbb5fa855aa22246b06c6a)