telemetry: Avoid breakage if user SSH config enables ControlMaster

telemetry's cros_interface cannot handle a persistent master connection;
it will hang until the master connection exits.  Explicitly disable
ControlMaster on the command line, to override any user
configuration enabling it.

Observed when attempting to use run_gpu_test.py on Chrome OS with a
~/.ssh/config in the chroot that enabled ControlMaster and
ControlPersist; each invocation of ssh by run_gpu_test.py hung until the
timeout specified in ControlPersist expired.  With this change,
run_gpu_test.py completes successfully despite that user configuration.

Review URL: https://codereview.chromium.org/399263002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284001 0039d316-1c4b-4281-b951-d872f2087c98

tools/telemetry/telemetry/core/backends/chrome/cros_interface.py

@@ -104,7 +104,8 @@ class CrOSInterface(object):
                       '-o StrictHostKeyChecking=no',
                       '-o KbdInteractiveAuthentication=no',
                       '-o PreferredAuthentications=publickey',
-                      '-o UserKnownHostsFile=/dev/null']
+                      '-o UserKnownHostsFile=/dev/null',
+                      '-o ControlMaster=no']
 
     if ssh_identity:
       self._ssh_identity = os.path.abspath(os.path.expanduser(ssh_identity))