run_tests.sh improvements

1. Run shotgun's tests

2. Write XUnit data to separate files

   Since now, the `run_tests.sh` writes XUnit XMLs as separate files.
   Both filenames and paths could be overridden by setuping next env variables:

   * NAILGUN_XUNIT                  (default: nailgun.xml)
   * FUELCLIENT_XUNIT               (default: fuelclient.xml)
   * FUELUPGRADE_XUNIT              (default: fuelupgrade.xml)
   * FUELUPGRADEDOWNLOADER_XUNIT    (default: fuelupgradedownloader.xml)
   * SHOTGUN_XUNIT                  (default: shotgun.xml)

   Example:

       NAILGUN_XUNIT="~/reports/nailgun.xml" ./run_tests.sh --with-xunit

Closes-Bug: #1304361
Closes-Bug: #1311073

Change-Id: I60e7733ed4c85e6decb29635d55e3ec8befa8ee1
Merge "run_tests.sh improvements"