Steps 
[1] Add a non-block skylab_test_push to test_push which runs the test_push suite with --create-and-return. Once the suite completes, verifies expected test results against given TKO instance.
[2] Make step blocking.
[3] Add powerwash test to skylab_test_push.

We got a single test to pass in the staging lab, even though the overall suite is still failing due to  issue 868528 : https://pantheon.corp.google.com/storage/browser/chromeos-autotest-results/swarming-3ef63fc0f5e24211

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/autotest/+/dd9071f9c40df8963be73a0527c53e633944cfb3

commit dd9071f9c40df8963be73a0527c53e633944cfb3
Author: Prathmesh Prabhu <pprabhu@chromium.org>
Date: Tue Jul 31 14:48:34 2018

Add skylab test_push.

BUG= chromium:867282 
TEST=Run skylab test_push manually.

Change-Id: If51ceb5e622c0d372998de8a8b6ee1eb51ba3668
Reviewed-on: https://chromium-review.googlesource.com/1152560
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>

[add] https://crrev.com/dd9071f9c40df8963be73a0527c53e633944cfb3/venv/skylab_staging/test_push.py
[add] https://crrev.com/dd9071f9c40df8963be73a0527c53e633944cfb3/venv/skylab_staging/errors.py
[add] https://crrev.com/dd9071f9c40df8963be73a0527c53e633944cfb3/venv/skylab_staging/__init__.py
[add] https://crrev.com/dd9071f9c40df8963be73a0527c53e633944cfb3/bin/skylab_test_push
[add] https://crrev.com/dd9071f9c40df8963be73a0527c53e633944cfb3/venv/skylab_staging/swarming.py

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/chromeos-admin/+/7637fd518963ac78a1a137bff4cd6fea1e01c03a

commit 7637fd518963ac78a1a137bff4cd6fea1e01c03a
Author: Prathmesh Prabhu <pprabhu@chromium.org>
Date: Tue Jul 31 19:10:27 2018

skylab_test_push is now green, so let's block staging_test on it:
https://chrome-internal-review.googlesource.com/c/chromeos/chromeos-admin/+/665589

issue 866640 takes care of the last remaining bit -- site_utils/test_push.py should not unilaterally decide to push the prod branches on autotest, chromite.

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/chromeos-admin/+/64dceaa9c979c1bce9b941b3cc1c1b94bf5b0c5f

commit 64dceaa9c979c1bce9b941b3cc1c1b94bf5b0c5f
Author: Prathmesh Prabhu <pprabhu@chromium.org>
Date: Wed Aug 22 17:17:27 2018

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/chromeos-admin/+/97c197f311eee21d03c9d248d631ef1bc8254c44

commit 97c197f311eee21d03c9d248d631ef1bc8254c44
Author: Prathmesh Prabhu <pprabhu@chromium.org>
Date: Wed Aug 22 17:38:50 2018

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/chromeos-admin/+/8e40335359b1e10cf6436e1dd0055dfa042c34e0

commit 8e40335359b1e10cf6436e1dd0055dfa042c34e0
Author: Prathmesh Prabhu <pprabhu@chromium.org>
Date: Wed Aug 22 22:57:56 2018

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/chromeos-admin/+/d60c4d2b2bee4403fcda1ab2bd3a02d9b8be96fc

commit d60c4d2b2bee4403fcda1ab2bd3a02d9b8be96fc
Author: Congbin Guo <guocb@google.com>
Date: Thu Aug 23 20:10:27 2018

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/chromeos-admin/+/4353bcae667ee62b52e9da124182858f92768c12

commit 4353bcae667ee62b52e9da124182858f92768c12
Author: Congbin Guo <guocb@google.com>
Date: Thu Aug 23 20:20:10 2018

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/chromeos-admin/+/4ba72fe04ddadb9d4db465c99fffc4b35435e739

commit 4ba72fe04ddadb9d4db465c99fffc4b35435e739
Author: Prathmesh Prabhu <pprabhu@chromium.org>
Date: Fri Aug 24 01:36:30 2018

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/chromeos-admin/+/b7a0a359b2334d7160efd6f658377546328c98a1

commit b7a0a359b2334d7160efd6f658377546328c98a1
Author: Prathmesh Prabhu <pprabhu@chromium.org>
Date: Fri Aug 24 02:19:47 2018

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/chromeos-admin/+/349e98a401c78991c65d88ff861e05b756be1f07

commit 349e98a401c78991c65d88ff861e05b756be1f07
Author: Prathmesh Prabhu <pprabhu@chromium.org>
Date: Fri Aug 24 02:59:07 2018