[google|compute] Query global projects when get/list'ing compute images.

    This patch refactors some of the code that would query global
    projects when get'ing images. It makes the list of global projects
    a constant on the Images collection, so that both 'get' and 'list'
    can use the same list of projects to query.
    
    Also, when bootstrapping/create'ing a server, the validation of
    the specified image name is done by trying to 'get' the image
    instead of 'save'ing it.