Add time stamp to the captured tempest log

nosetests capturing the log output and displaying the log related to
ERRORed and FAILed test cases at the end of the test suite.
Without time stamp it is very difficult to find the correlation between the
tempest log and other log files...

Change-Id: I74e692548aff6e08128e9780cff1ba6e21040630
Merge "Add time stamp to the captured tempest log"