Add message queue switching through RequestContext

This adds message queue connection information to the RequestContext
which can be used by nova-api to communicate with a targeted cell
message queue with each query.

A function 'get_cell_client' can be called in rpc functions to enable them
to use message queue transport information from a RequestContext. The
function creates a rpc client object dynamically if message queue
connection information is found in the RequestContext and falls back on
the default rpc client.

Example usage:

    def get_cell_client(self, context):
        return rpc.get_cell_client(context, self.client)

    def build_and_run_instances(self, ctxt, instance, host, image, ...)
        cctxt = self.get_cell_client(ctxt).prepare(...)
        cctxt.cast(...)

Implements blueprint cells-mq-connection-switching

Change-Id: Idef670d5b73c9cef8501a0593eccd785b708bd2b

nova/rpc.py

@@ -65,6 +65,20 @@ TRANSPORT_ALIASES = {
 }
 
 
+def get_cell_client(context, default_client):
+    """Get a RPCClient object based on a RequestContext.
+
+    :param context: The RequestContext that can contain a Transport
+    :param default_client: The default RPCClient
+    """
+    if context.mq_connection:
+        return messaging.RPCClient(
+                context.mq_connection, default_client.target,
+                version_cap=default_client.version_cap,
+                serializer=default_client.serializer)
+    return default_client
+
+
 def init(conf):
     global TRANSPORT, NOTIFICATION_TRANSPORT, LEGACY_NOTIFIER, NOTIFIER
     exmods = get_allowed_exmods()
@@ -185,6 +199,14 @@ def get_versioned_notifier(publisher_id):
     return NOTIFIER.prepare(publisher_id=publisher_id)
 
 
+def create_transport(url):
+    exmods = get_allowed_exmods()
+    return messaging.get_transport(CONF,
+                                   url=url,
+                                   allowed_remote_exmods=exmods,
+                                   aliases=TRANSPORT_ALIASES)
+
+
 class LegacyValidatingNotifier(object):
     """Wraps an oslo.messaging Notifier and checks for allowed event_types."""