mon: OSDMonitor: Make 'osd pool rename' idempotent

'ceph osd pool rename' takes two arguments: source pool and dest pool.
If by chance 'source pool' does not exist and 'destination pool' does,
then, in order to assure it's idempotent, we want to assume that if
'source pool' no longer exists is because it was already renamed.

However, while we will return success in such case, we want to make sure
to let the user know that we made such assumption.  Mostly to warn the
user of such a thing in case of a mistake on the user's part (say, the
user didn't notice that the source pool didn't exist, while the dest did),
but also to make sure that the user is not surprised by the command
returning success if the user expected an ENOENT or EEXIST.

Fixes: #6635

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>