deploy_chrome: Add --private_key option

This allows using deploy chrome on test devices that have an ssh key other than the publicly
available test key.

BUG=778274
TEST=Replaced test key on a CrOS device and successfully deployed.
Change-Id: I5bdb032e0ed621758fca83c9d9c0309d5764ba40
Reviewed-on: https://chromium-review.googlesource.com/737552
Commit-Ready: Adrian Elder <aelder@chromium.org>
Tested-by: Adrian Elder <aelder@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/deploy_chrome_unittest.py b/scripts/deploy_chrome_unittest.py
index 872ed8e..7a8f82c 100644
--- a/scripts/deploy_chrome_unittest.py
+++ b/scripts/deploy_chrome_unittest.py
@@ -89,6 +89,12 @@
     options = _ParseCommandLine(argv)
     self.assertEqual(options.mount_dir, '/foo/bar/cow')
 
+  def testSshIdentityOptionSetsOption(self):
+    argv = list(_REGULAR_TO) + ['--private-key', '/foo/bar/key',
+                                '--board', 'cedar',
+                                '--build-dir', '/path/to/nowhere' ]
+    options = _ParseCommandLine(argv)
+    self.assertEqual(options.private_key, '/foo/bar/key')
 
 class DeployChromeMock(partial_mock.PartialMock):
   """Deploy Chrome Mock Class."""