jsonrpc: switch to the simplejson module

During testing, we discovered that on RHEL 6.x
and Centos 6.x we unnecessarily lose performance
because the JSON module shipped with python 2.6
does not include speedups found in python 2.7
and onwards.

The standard library module is based on the
simplejson package, and RHEL 6.x/Centos 6.x already
provide an updated simplejson rpm.

Then, this patches makes yajsonrpc use simplejson,
to recover speed at nearly zero cost.

To ease development and deployment, this patch
always make VDSM require simplejson, even
on platforms shipping a modern enough python
distribution.

Change-Id: Ic17aa6d75cef933ee36e002c03732aab14b39648
Bug-Url: https://bugzilla.redhat.com/1155146
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35024
Reviewed-by: Piotr Kliczewski <piotr.kliczewski@gmail.com>
Reviewed-by: Dima Kuznetsov <dkuznets@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35290
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>