mon: enforce FEATURE_CEPHX_AUTH_AES256K for AES256KRB5 usage

This commit introduces the `FEATURE_CEPHX_AUTH_AES256K` monitor feature
and ensures it is strictly supported by the quorum before allowing the
configuration or usage of AES256KRB5 ciphers.

Change-Id: f0c27460-8ed5-4691-b751-27e20baa1bc3
Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>