Generate one single report for Rally testcases

Generate one single report for Rally testcases

It eases sharing one single report including all task results.
For the time being, rally_jobs only generates neutron.html.
Then it only adds one report for rally_full and rally_sanity.

We may consider generating only one json file but it asks for
a refactoring (result parsing) which can be done in a second
change.

Change-Id: I8fa75ae80750336950f355c2aedc3dd4dc621cc5
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit 2df7a0728d687de9a24d8432ef30bbd2ecd771c8)