Add new attributes for common rpc configuration

We need provide ability to configure the following rpc related
attributes:
rpc_thread_pool_size
rpc_conn_pool_size
rpc_response_timeout
There is the similar changes on cinder:
https://review.openstack.org/#/c/58189/

Change-Id: I93765f798b4c74727b8e4e962749e37468080c35