Update GCE source_image to debian-7-wheezy-v20140415
[GCE] gsutil update fails in newer images, use gcloud

    googlecompute: It looks like you are trying to run "/usr/local/bin/../share/google/google-cloud-sdk/bin/bootstrapping/gsutil.py update".
    googlecompute: The "update" command is no longer needed with the Cloud SDK.
    googlecompute: To update, run: gcloud components update
builder/googlecompute: added support for all standard VM images

Also fixed the error message to be clear that the image was not found in any of
the projects that we attempted to search, rather than keep the error message
from the last project, which may be confusing to users.
Added CoreOS, OpenSUSE, and Windows projects for searching images.
Merge pull request #1292 from maestrodev/gcloud

[GCE] google compute builder is broken, gsutil update fails in newer images
Merge pull request #1350 from mbrukman/googlecompute-driver-vm-images

builder/googlecompute: added support for all standard VM images
Rename "Gsutil" to "Gcloud" now that we're updating "gcloud" and not "gsutil".

Also renamed files accordingly.
Merge pull request #1351 from mbrukman/googlecompute-gsutil-to-gcloud

builder/googlecompute: rename "Gsutil" to "Gcloud" now that we're updating "gcloud" and not "gsutil".
Update GCE source_image to debian-7-wheezy-v20140718
Merge pull request #1216 from playlist-media/fix-gce-image-name

Update GCE source_image in the docs