Expose needed services on the control socket

For swarm mode to function without exposing a TCP port, we need services
such as the dispatcher and node CA to be exposed on the control socket
(i.e. a unix socket). This commit changes the manager to expose those
services, and changes the raft proxy to inject some information into the
context when calling the handler directly that identifies the local
node. The authorization code in "ca" is updated to check for this
information on the context and make use of it, instead of returning an
error from RemoteNode. Also, the CA server now renewing a certificate
over the control socket.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>