change collector_workers to [collector]workers
change collector_workers to [collector]workers

We have deprecated collector_workers, let's update it in devstack as
well.

Change-Id: I914d34998ca357b1dea117229fbee494509d8f57
Closes-Bug: #1505555

maestro-hybrid-cloud/ceilometer,ityaptin/ceilometer,eayunstack/ceilometer,ityaptin/ceilometer,fabian4/ceilometer,maestro-hybrid-cloud/ceilometer,idegtiarov/ceilometer,openstack/ceilometer,fabian4/ceilometer,eayunstack/ceilometer,openstack/ceilometer,isyippee/ceilometer,idegtiarov/ceilometer,isyippee/ceilometer