Adds html result generator for tempest runs

* Adds "rally verify results --html --output-file output_to_html_file" flag to generate html result from
  tempest runs
* json2html generates html report similar to subunit2html using raw json
  from rally database
* Use mako templates for html generation

Change-Id: Ib390fa02aba4696bacaa4e4ec8b8c2481a54b0d2
Implements: blueprint tempest-subunit-to-json
Merge "Adds html result generator for tempest runs"