librbd: better error when unprotect fails on unprotected snap

This will show up on the command line and logs, making it more
clear than EINVAL.

Fixes #6851 and #4045
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #1010 from dachary/wip-mon-keyring

implement --key as an alternative to --keyring when ceph-mon --mkfs

Reviewed-by: Sage Weil <sage@inktank.com>
Merge pull request #1027 from jdurgin/wip-unprotect-error

librbd: better error when unprotect fails on unprotected snap

Reviewed-by: Sage Weil <sage@inktank.com>
mon: tests for ceph-mon --mkfs

* auth none must not require a keyring
* --key can be used as an alternative to --keyring
* --mkfs is idempotent
* the --mon-data directory is created if it does not exist
* auth ceph requires --keyring

Signed-off-by: Loic Dachary <loic@dachary.org>