Use jsonutils instead of stdlib json

Use jsonutils instead of stdlib json

jsonutils provides multiple benefits in comparison to pure stdlib json
(like using simplejson on Python 2.6).

Similar patch was already merged before [1], but since it lacked hacking
rule to enforce jsonutils usage, new occurrences of stdlib json module
usage were introduced.

This patch switches all the code to using jsonutils and adds a hacking
rule to enforce the rule.

The hacking rule requires that jsonutils module does not mimic as 'json'
thru using import renames, so the code was updated not to rename the
module when doing import.

The hacking rule was shamelessly copied from the corresponding nova
review [2].

[1]: https://review.openstack.org/#/c/99760/
[2]: https://review.openstack.org/111296/

Change-Id: Ie7a5bb76445e15cde9fbf9ff3d2101a014637b37