Do not enable cinder-volume on non-cinder nodes

Only cinder nodes have "cinder" volume group created during
provisioning, when cinder-volume service is deployed with Cinder LVM
backend on a node that doesn't have a "cinder" VG, volume creation on
such nodes is impossible.

is_cinder_node variable is no longer used in osnailfyfacter so the logic
to generate its value can be safely removed.

Change-Id: I1d4eab2435cc87625733674d26e0c30cc593dd6a
Closes-bug: #1283035
Merge "Do not enable cinder-volume on non-cinder nodes"