Add support for omaha in update_device.py

Automatically detect if the device is running an update_engine with
USE_OMAHA or not, and use different command based on that.

Made the http server serving payload a fake omaha server as well.

Added support for overriding the payload public key on the device.

Bug: 27316596
Test: update a device that uses omaha
Change-Id: I230d58ff105189dc8a624d46ee79cbe90c5f09b0