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

Change-Id: Ib3dc0b713ed90396919feba018772243b3b9c90f
Closes-Bug: 1314129
Merge "On Python <= 2.6, use simplejson if available"