mon: pool create erasure implicit ruleset creation

mon: pool create erasure implicit ruleset creation

If the crush_ruleset parameter is missing, set it to the pool name.

If the crush_ruleset parameter is set to a name that does not match any
of the existing rulesets, create one using the pool creation parameters.

If the ruleset exists and is in the pending map or if the ruleset was
just created (meaning it exists in the pending map), the
prepare_pool_crush_ruleset method returns EAGAIN so that the pool
creation message is retried after the pending map is proposed.

If the ruleset exists, it is used to initialize the newly created pool,
as before.

http://tracker.ceph.com/issues/7571 fixes #7571

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