mon: pool set <pool> crush_ruleset must not use rule_exists

Implement CrushWrapper::ruleset_exists that iterates over the existing
rulesets to find the one matching the ruleset argument.

ceph osd pool set <pool> crush_ruleset must not use
CrushWrapper::rule_exists, which checks for a *rule* existing, whereas
the value being set is a *ruleset*. (cherry picked from commit
fb504baed98d57dca8ec141bcc3fd021f99d82b0)

A test via ceph osd pool set data crush_ruleset verifies the ruleset
argument is accepted.

http://tracker.ceph.com/issues/8599 fixes: #8599

Backport: firefly, emperor, dumpling
Signed-off-by: John Spray <john.spray@inktank.com>
Signed-off-by: Loic Dachary <loic@dachary.org>