Create SecurityGroupServerRpcApi and add some docs

Create SecurityGroupServerRpcApi and add some docs

Create SecurityGroupServerRpcApi, which is intended to replace
SecurityGroupServerRpcApiMixin over time.  The new class is the same
code, but used as a standalone class.  This is needed to be able to
eventually move this API into a messaging namespace.

In passing, add docstrings that show where to find the corresponding
other side of this interface in the code base.

Part of blueprint rpc-docs-and-namespaces.

Change-Id: If959b2f8bf21b4963bf6876a3eedc0a4b06c6400