Bug 1103031 - switch over to using base-test and base-build images

* Moved the base image to be base-build and updated the builder to use that as a base image
* Copied the base and tester image from gecko-images
* Downgraded to Centos 6 because of glib version issues between Centos 6 and Ubuntu 12.04 causing b2g not to start up.  
* Updated some of the packages to reflect downgrading to Centos 6
* Updated hg and git so it is now using the latest packages available from the releng site
* Updated env path to reflect through found in the buildbot scripts and that other scripts like gittool.py expect in this new configuration

Build and tested with b2g desktop, mulet, and emulator:

http://docs.taskcluster.net/tools/task-graph-inspector/#1DKajVqnQGagaj8ci9ifQQ
http://docs.taskcluster.net/tools/task-graph-inspector/#MX26lOXhS6mN-F2rCi1WNw//0
http://docs.taskcluster.net/tools/task-graph-inspector/#e2-V98OvRaK2lVwzbeugcg//0

ah, good catch.  I just reused the test-setup.sh from gecko-images and didn't notice that piece.  I don't think we have any test tasks that upload an artifact, but we probably will soon.  mozharness doesn't know about /home/worker/artifacts, and we should change that.  I'll file a bug to change that for our test tasks.  I'm not completely clear on how or what mozharness logs.

Fixed in 193356f