Use get_rpc_transport instead of get_transport

With the added get_rpc_transport() function in oslo.messaging [1]
and the recent version bump of oslo.messaging to 5.24.2 [2], we can
safely replace get_transport() with get_rpc_transport() which is much
cleaner.

[1] https://review.openstack.org/#/c/454194/
[2] https://review.openstack.org/#/c/470144/6

Change-Id: If38ffb3fec088b94f39626ed7fa27b343e8dfbea

TODO.rst

@@ -38,8 +38,6 @@ POLICY
 MISC
 ----
   - Remove all usage of enforce_type when calling oslo.config set_override().
-  - Change messaging.get_transport to get_rpc_transport
-  - 
 
 MIDDLE PRIORITY
 ===============

senlin/common/messaging.py

@@ -74,7 +74,7 @@ def setup(url=None, optional=False):
     if not TRANSPORT:
         exmods = ['senlin.common.exception']
         try:
-            TRANSPORT = messaging.get_transport(
+            TRANSPORT = messaging.get_rpc_transport(
                 cfg.CONF, url, allowed_remote_exmods=exmods)
         except messaging.InvalidTransportURL as e:
             TRANSPORT = None