Fix Vault K/V API compatibility.
Fix Vault K/V API compatibility.

Starting from version 0.10.0, HashiCorp Vault has a different
API for the Key/Value Secrets Engine. This fix implements both
the new API support and the fallback for the legacy one.

Fixes https://bugs.launchpad.net/castellan/+bug/1788375

Change-Id: I7fed7b5091440dae15551d83f0ee0895651e47bf
Signed-off-by: Moises Guimaraes de Medeiros <38931a700a64030c6e1883420cbba039aadcb37a@redhat.com>

openstack/castellan,openstack/castellan