Add ability to compare two verification results

Currently there is no built-in way in Rally to see the differences
between two verification runs. This feature adds a compare sub-command
to rally verify that takes two verification IDs as input and output the
differences between two verifications in the desired format, html, csv,
json or pprint. Output may be printed to std out or directed to file
destination specified.

Tasks
- Add compare sub-command to rally verify.
- Add ability to specify output format and file name of comparison
  report.
  Initial support will allow for output to either HTML, pretty print
  or CSV.
- Create rally/verification/verifiers/tempest/diff.py that handles all
  the comparing and output of results.
- Create required mako template (compare.mako) and renderer
  (compare2html.py) class for HTML output.
- Added percentage threshold argument for determining execution time
  differences

Change-Id: I0276e431d7d1ad89b1b28306cd0e0ea7e1174217
Implements: blueprint tempest-compare
Merge "Add ability to compare two verification results"