Add an rpc API common to all services.

This patch adds an rpc API that is exposed by all services.  The methods
in this API exist in their own namespace and are versioned independently
of the main API for the service.

The first method for this API is a simple ping() method.  This method
exists in the conductor rpc API already, and could be more generally
useful.  Other methods will be added in later patches.

The base rpc API will be exposed from all services automatically unless
they override the create_rpc_dispatcher method in the base manager
class.  All services need to pass a service_name into the base manager
constructor.  Some services already did this, but now it's needed for
all of them.

Implements blueprint base-rpc-api.

Change-Id: I02ab1970578bc53ba26461b533d06d1055c2d88e
Merge "Add an rpc API common to all services."