autotest: switching full release test to use AFE

With this change, full_release_test can schedule autoupdate_EndToEndTest
jobs via an Autotest Front-End (AFE).  This includes:

* When using the AFE, we fetch and specialize the test's control script
  with test-specific arguments.

* Now requiring an absolute path to a devserver checkout, so
  a non-chrooted test execution can spawn a private Omaha/devserver
  instance. A commented out definition appears in global_config.ini, and
  should be overridden for specific environments in shadow_config.ini.

Other minor changes:

* Now inferring the absolute location of imported modules.

* Now enforcing a timeout on HTTP requests when reading host logs from
  the devserver (30 secs). Also, HTTP exceptions are caught and handled,
  not causing a test to crash.

* Eliminated the dev_mode option (not needed).

* Removed redundant str() conversions.

BUG=chromium-os:33762
TEST=tests scheduled via AFE, executing to completion

Change-Id: I3ef827b979faef7ba56d26664536de7b0c91cfc1
Reviewed-on: https://gerrit.chromium.org/gerrit/38841
Tested-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Alex Miller <milleral@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Ready: Gilad Arnold <garnold@chromium.org>
diff --git a/global_config.ini b/global_config.ini
index 95bb445..f588567 100644
--- a/global_config.ini
+++ b/global_config.ini
@@ -157,3 +157,7 @@
 rpm_sentry_password: fake_password
 rpm_frontend_uri: http://chromeos-rpmserver1.cbf.corp.google.com:9999
 rpm_recovery_boards: stumpy,kiev
+
+# Path the devserver source tree, used for spawning devserver from autoserv in
+# some of the tests. Please override in local shadow config file.
+#devserver_dir: /path/to/src/platform/dev