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>
(cherry picked from commit d02d46e25080d5f7bb8ddd4874d9019a078b816b)

Conflicts:
	src/mon/OSDMonitor.cc
Merge pull request #1963 from dachary/wip-8599-ruleset-dumpling

mon: pool set <pool> crush_ruleset must not use rule_exists (dumpling)

Reviewed-by: Sage Weil <sage@inktank.com>
osd: introduce simple sleep during scrub

This option is similar to osd_snap_trim_sleep: simply inject an optional
sleep in the thread that is doing scrub work.  This is a very kludgey and
coarse knob for limiting the impact of scrub on the cluster, but can help
until we have a more robust and elegant solution.

Only sleep if we are in the NEW_CHUNK state to avoid delaying processing of
an in-progress chunk.  In this state nothing is blocked on anything.
Conveniently, chunky_scrub() requeues itself for each new chunk.

Backport: firefly, dumpling
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit c4e8451cc5b4ec5ed07e09c08fb13221e31a7ac6)
hack: print scrub thread id to log at osd debug level 2