Bug 1306421 - Disable proxxy in Jenkins QA environment; r=whimboo

Bug 1306421 changed the behavior of proxxy in an attempt to make it
more consistent. As a result of that, it enabled proxxy in places
where it wasn't used before. One of them was the Jenkins QA environment
which runs update UI tests. Unfortunately, that environment
doesn't have connectivity to proxxy.

This commit simply blanks out the proxxy config in the Jenkins QA
environment, effectively disabling proxxy.

MozReview-Commit-ID: 3A37VKEGQ49

--HG--
extra : rebase_source : ac63444f66909079297defcf5e0e9ff10b66e510

testing/mozharness/configs/firefox_ui_tests/qa_jenkins.py

@@ -13,4 +13,7 @@ config = {
     'download_minidump_stackwalk': True,
     'download_symbols': 'ondemand',
     'download_tooltool': True,
+
+    # Disable proxxy because it isn't present in the QA environment.
+    'proxxy': {},
 }