Monitor: Only sleep non-success, nonfirst loop iterations

Move the sleep() statements in the wait_for.. methods so they are (1)
only run after the first loop iteration and (2) only if there are
still some metrics we are waiting for.

Change-Id: I4e31a0c477aa3b171c569834050a20b0b97805be
Reviewed-on: http://review.couchbase.org/39945
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>