Use RPC directly for software config operations

This change replaces heatclient REST calls with RPC calls when
creating, fetching or deleting software configs.

All resources can now request and RPC client with self.rpc_client()
which will create and memoize an EngineClient instance.

Change-Id: Id23749e672fd4154e3d9986e22b46fc038579d8e
Partial-blueprint: software-config-trigger