Bug 1055051: Switch to Ubuntu 12.04 + mozilla mesa packages to avoid ref...

...test failures

With the existing docker "gecko-tester" image, 300+ reftests fail with minor differences. This is caused by a known problem in the mesa libs and a Mozilla fix is available in the form of mesa packages distributed to test slaves via puppetagain.

This commit changes all of the docker images (starting from gecko-base) to Ubuntu 12.04 (instead of 13.10). 12.04 is currently in use on the test slaves, and the mesa packages are for 12.04. On gecko-tester, we install the Mozilla mesa packages and a few others to make the environment more like the current test slaves. This change triggered a problem in pulseaudio, causing pulseaudio to fail to load certain modules, pulseaudio to fail to start, and certain media mochitests to fail, but all of those problems are easily avoided by installing a custom pulseaudio configuration file to disable the troublesome modules.

With this commit, I have had good success running all b2g-desktop tests (mochitest, reftest-sanity) and b2g-emulator (mochitest, crashtest, reftest, and xpcshell tests) to completion without any unexpected results.