Sync oslo-incubator rpc	module

Sync oslo-incubator rpc module with oslo-incubator commit
f8f6fd46b3b9264c21c0ee5547a4e8236cb257ac

Updates consist in using six.iteritem and fixing some typos.

Remove securemessage.py, which in not used anywhere, and dropped from
oslo-incubator since commit f88d59ab8a456e7d2d5a22fc5591491d2f85bdf4

$ python update.py --nodeps --base keystone --dest-dir ../keystone/ --modules rpc
$ git rm keystone/openstack/common/rpc/securemessage.py

Change-Id: I890cf1ce89ce5946522f7891be71a3e58ecc1af3
Merge "Sync oslo-incubator rpc	module"