Re-run failed Tempest tests

Registered by Castulo J. Martinez

Add the ability to re-run the tempest tests that failed in the last test execution.

Sometimes Tempest tests fail due to a special temporary condition in the environment, for example a temporary slowness in the system which causes some tests to timeout, in such cases it is very useful to be able to re-execute those tests that failed from the last test execution so you can determine if it was a temporary failure or something that is failing consistently.

Test repository (testr) already provides such functionality for test execution using the following command:
# Run the failed tests
$ testr run --failing

So I am proposing to extend this functionality already provided by testr from within Rally giving the user the oportuity to use it as an attribute of verify start, something like:
$ rally verify start --failing

Blueprint information

Status:
Complete
Approver:
Boris Pavlovic
Priority:
Medium
Drafter:
Castulo J. Martinez
Direction:
Approved
Assignee:
Castulo J. Martinez
Definition:
Approved
Series goal:
None
Implementation:
Implemented
Milestone target:
None
Started by
Castulo J. Martinez
Completed by
Castulo J. Martinez

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Work Items

Work items:
- Implement the `rally verify --failing` command: DONE
- Add unit tests: DONE

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