ceph: set return limit
ceph: set return limit

when grabbing metrics to process, we only grab a chunk. there's no
point grabbing uncapped amount and taking small chunk near front.

Change-Id: I9ac64037705f08a3133f6d845ced516aab50d40f

leandroreox/gnocchi,sileht/gnocchi,gnocchixyz/gnocchi,gnocchixyz/gnocchi,leandroreox/gnocchi,sileht/gnocchi