Use command line call instead librbd call

When using python binding of librbd we are limiting our self to
sequencial operation to Ceph backend b/c eventlet can't monkey
patch C code, in another hand a command line call work just fine
with eventlet.

Beside performance consideration we are chaning this back to
command line call from https://review.openstack.org/#/c/30792
b/c we notice that sometime a volume delete call may hang librbd
side (No idea why yet :( ) which put the cinder-volume process in
a hanging state (no progress) and so the process must be restarted.