Performance gate jobs

Create gate job which will check performance of neutron using Rally.
Publish results in comments under CR. (sample: http://logs.openstack.org/73/97473/4/check/check-rally-dsvm-neutron-rally/91b538f/rally-plot/results.html.gz )

Create directory "rally-scenarios" in project's root to make it easy to write any scenario and see results after uploading a CR.

Create default scenario which will be used for every CR.

Also Rally plugins may be written and stored in "rally-scenarios/plugins" for benchmarking some exlusive stuff.

Please note: CRs with new scenario/plugins may be uploaded only for getting results of benchmarks, and there is no need to merge it.