Installing MIQ on Google Compute Engine - simplification

[kuklis]

There is no need to download and upload the storage file as it is already stored in Google Storage as public. Steps in 1.1 and 1.2.3-1.2.5 are unnecessary. Instead you can jump to creating the image, just provide the bucket and storage file name in "Cloud Storage File" field, ie:
manageiq/gaprindashvili-1-rc1.tar.gz
See https://cloud.google.com/compute/docs/instances/create-start-instance#publicimage

[simaishi]

We're no longer uploading GCE images. Rather than changing this doc, we'll need to update Get Started doc so we don't instruct users to use pre-published images (ManageIQ/manageiq.org#794)