Run unit tests against a particular git branch

Solum runs unit tests always against the master
branch, which does not reflect the real dev flow.

This patch focuses on handling Github pull requests.
Solum trigger API is able to get a particular branch name
from request body sent by a Github webhook, and pass
it down to worker handler for running unit tests against
it.

Closes-bug: 1363249

Change-Id: I7ecdf626c99e16f9970ad4089020c95572c3da83