mon: provide emergency mechanism to use mon keyring

mon: provide emergency mechanism to use mon keyring

If they key is lost for the `mon.` credential, it's very inconvenient to get it
out of the "auth" database in the mon store. So, allow the operator to create a
new keyring for the mons and use it instead to get mons in quorum again.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>