[gke.py] Implement full deployment cycle.
[gke.py] Implement full deployment cycle.

Implement the full deployment lifecycle on top of "gke.py". This
includes building and uploading new images, generating new deployments,
switching to new deployments, rolling back to previous deployments, and
marking deployments as "stable".

TBR=vadimsh@chromium.org
BUG=None
TEST=local

Change-Id: Idb40be6898cb93c9aa78bbccfb98ef27e91bb51b
Reviewed-on: https://chromium-review.googlesource.com/626760
Commit-Queue: Daniel Jacques <8400006a9338a0893dca0101ad49838955bd6b2c@chromium.org>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py