Remove check_for_lock support in RPCClient

I added check_for_lock because I assumed it was enabled by default and
actively in use by Nova. However, it actually isn't used by Nova yet and
enabling spews a tonne of warnings.

It's a rather clunky API and there's a good chance we can design a
better API for it, so let's leave it out until we're ready to actually
start using it in Nova.

Related-Bug: #1063222
Change-Id: Ib890978398059f360cd0f3352f4755262b8111c6
Merge "Remove check_for_lock support in RPCClient"