Update completed tasks status to "finished"

After successfully finishing a benchmark task, Rally does not update its
status from "test_tool->benchmarking" to "finished". As a result, these
tasks get incorrectly stored in the database with the "test_tool->benchmarking"
status, which prevents them from being deleted according to user's requests.

The patch also adds the "failed" task status and sets this status in
case of an error occuring in the test engine 'with' block.

Closes-Bug: #1255980
Blueprint independent-deploy

Change-Id: I5088f24feb35459d850ae054a62a0544b765ba40
Merge "Update completed tasks status to "finished""