Changed Nagios check service_cinder_volume.

The Nagios check service_cinder_volume looks for a hard-coded number of processes of /usr/bin/cinder-volume owned by the user cinder. However, on the Qld cell's cinder-volume host, there are two such processes, and thus the Nagios alert is always critical.

I have edited the cinder::volume class to make this number of processes a variable (default value of 1, which can be changed in hiera with cinder::volume::processes).