RPC client lazy initialization

All *Command classes are instantiating for each cinder-manage command
execution. RPC client should be initialized for VolumeCommands usage.
Some custom RPC clients (oslo.messaging backends) could init RPC
connection after client initialization so it is no need to create RPC
client for every cinder-manage command invoke.

Related-Bug: #1348787
Change-Id: I7b3388dded6eeec972d9949538e1608c3a538734
Merge "RPC client lazy initialization"