On Python <= 2.6, use simplejson if available

Until Python 2.7, stdlib json module was not boosted with a C extension,
meaning bad performance. Try to use simplejson module if available.

RHEL 6.5 is one of the platforms that still ship Python 2.6.

The original idea of the patch belongs to Miguel Angel Ajo Pelayo @
mangelajo@redhat.com

Conflicts:
	openstack/common/jsonutils.py

Change-Id: Ib3dc0b713ed90396919feba018772243b3b9c90f
Closes-Bug: 1314129
(cherry picked from commit a6b2aecf3a75e4bdef493b645112a7944bced1e1)