Always import heat.rpc.api as rpc_api

heat.rpc.api was being imported as rpc_api in some modules and engine_api
in other modules.  Always import heat.rpc.api as rpc_api for consistency.

Change-Id: Ic84ca7fbac7a95eace7f2019c08ee20fa1bc8e5e
Merge "Always import heat.rpc.api as rpc_api"