rgw/kms/vault - relax configuration parsing for rgw_crypt_vault_secret_engine

rgw/kms/vault - relax configuration parsing for rgw_crypt_vault_secret_engine

To better manage forwards and backwards compatibility when using vault
transit for rgw object encryption (sse:kms); it is desirable to provide
parameters to control how this works.  It was more attractive to overload
the existing rgw_crypt_vault_secret_engine parameter for this purpose
than to invent one or more all-new parameters.

Additionally, the enum support in the configuration parser looks like
it ought to have helpful syntax checking functionality.  This is not so;
failure to provide a supported enum results in silently replacing that
with the default option, resulting in confusing and non-obvious behavior
that is not at all helpful.

This change removes the enum constraint on rgw_crypt_vault_secret_engine,
allowing for more useful messages from the rgw code, and the possibility
to also provide additional information on the same line.

Fixes: http://tracker.ceph.com/issues/48746
Signed-off-by: Marcus Watts <mwatts@redhat.com>
(cherry picked from commit 845dd67b3d0b5ee297171bba437797a18e8711ee)