Allow specifying project for source images in GCE

Within GCE, images may be shared across projects. Prior to this
commit, there was no way to inform the GCE builder that a source
image belonged to a specific project. This adds an optional
'source_image_project_id' key to the GCE builder config.
Merge pull request #1431 from mpage/gce-project-id-for-source-image

Allow specifying project for source images in GCE