[nailgun] Get a rid of json.loads/json.dumps from tests

[Ivan Kolodyazhny]

Almost all tests for REST API uses following code structure:
resp = self.app.post(
            reverse('TestHandler'),
            json.dumps({'test': 'test'}),
            headers=self.default_headers,
            expect_errors=True)
self.assertEquals(json.loads(resp.body), {'response': 'test'})

Calling json.loads/json.dumps should be moved to base class to make more clear and DRY code.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/125413

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/125413
Committed: https://git.openstack.org/cgit/stackforge/fuel-web/commit/?id=54c80d9b96448cf24570499e065b7bc7ffdf3023
Submitter: Jenkins
Branch: master

commit 54c80d9b96448cf24570499e065b7bc7ffdf3023
Author: Ivan Kliuk <email address hidden>
Date: Wed Oct 1 06:32:44 2014 +0300

    Refactoring JSON serialization of response object

        Swapping DRY code jsonutils.loads(webob.TestResponse.body) with
        webob.TestResponse.json_body property.

    Change-Id: I2f4e0cd3f1f8abc209e19491287f3630b1c4e80d
    Closes-Bug: #1304967