Removed RestClientXML class

After unifying RestClient class to handle both container
types - json and xml, class RestClientXML becomes redundant.

Change-Id: I1bcf5e4df130b8eee26d0abcf1741ba3ea8f3d00
Partially-implements: bp refactor-rest-client
Fix stress runner exit issues

1. The stress runner in parallel run, regardless to the test failures
   returned with 0 (EXIT_SUCCESS) exit status.
2. Stress runner did not stopped run in serial execution, when the stop
   on first failure was enabled.

Change-Id: Ifb71c58a2f676f8a3427160dbc5cfeab61d13b56
Merge "Fix stress runner exit issues"
Merge "Removed RestClientXML class"
Merge commit 'refs/changes/20/75120/6' of git://gold.eng.hq.xensource.com/git/github/openstack/tempest into tmpbranch