Add parameter to customize Qpid receiver capacity

In Qpid rpc backend, receiver capacity is 1 and is hardcoding.
User can not customize it. This patch adds a parameter to make
user can specify receiver capacity in conf file.

- Reference about Receiver Capacity(Prefetch) from Qpid Doc
By default, receiver requests the next message from the server
in response to each fetch call, resulting in messages being
sent to the receiver one at a time. As in the case of sending,
it is often desirable to avoid this roundtrip for each message.
This can be achieved by allowing the receiver to prefetch messages
in anticipation of fetch calls being made. The receiver needs to
be able to store these prefetched messages, the number it can hold
is controlled by the receivers capacity.

DocImpact

Change-Id: I966b512aba6bdd8e9c5cf65cae01bfd21f04f330
Closes-Bug: #1353914

oslo/messaging/_drivers/impl_qpid.py

@@ -69,6 +69,9 @@ qpid_opts = [
     cfg.BoolOpt('qpid_tcp_nodelay',
                 default=True,
                 help='Whether to disable the Nagle algorithm.'),
+    cfg.IntOpt('qpid_receiver_capacity',
+               default=1,
+               help='The number of prefetched messages held by receiver.'),
     # NOTE(russellb) If any additional versions are added (beyond 1 and 2),
     # this file could probably use some additional refactoring so that the
     # differences between each version are split into different classes.
@@ -125,6 +128,7 @@ class ConsumerBase(object):
         """
         self.callback = callback
         self.receiver = None
+        self.rcv_capacity = conf.qpid_receiver_capacity
         self.session = None
 
         if conf.qpid_topology_version == 1:
@@ -178,7 +182,7 @@ class ConsumerBase(object):
     def _declare_receiver(self, session):
         self.session = session
         self.receiver = session.receiver(self.address)
-        self.receiver.capacity = 1
+        self.receiver.capacity = self.rcv_capacity
 
     def _unpack_json_msg(self, msg):
         """Load the JSON data in msg if msg.content_type indicates that it