Add argparse for RHEL6

python2.6 doesn't have argparse, required by outfilter.py.  This
package is a dependency on cloud-init, so this problem is often masked
in many testing environments.

Change-Id: Ia564f1b8a7b540faa7bbdbd1a1a290c591901682
Merge "Add argparse for RHEL6"
Merge branch 'master' of https://github.com/openstack-dev/devstack