Scope dhcp rpc api using a messaging namespace

This patch updates the rpc API used by the DHCP agent to make calls
back into the neutron plugin to use the 'dhcp' namespace instead of
the default namespace.  The reason is that this API is exposed over
the 'q-plugin' topic along with several other interfaces.  Without the
use of namespaces, all of the interfaces are effectively treated as
one by oslo.messaging.  When a namespace is used, the interface can be
versioned independently and when a method is called, the only class
considered for fulfilling the request is the one that claims to
implement the 'dhcp' namespace.

While we're here, add documentation to both the client and server side
of this interface that indicates where the other side is located.

Part of blueprint rpc-docs-and-namespaces.

Change-Id: I9e56aa34fc560ae3fc749c51788436e32179d0a1
Merge "Scope dhcp rpc api using a messaging namespace"