fixes #3832 - foreman supplies user for GCE images, fog expects username

(cherry picked from commit 51908af77554e588e85f7a6f73f1ce3e15aa4157)
fixes #3717 - pin fog to 1.15.x

(cherry picked from commit 4cc18563a29cf2e94e0e1a92dfdae14aea6b97a2)

Conflicts:
	bundler.d/fog.rb
	foreman.spec
Fixes #3653: Always set the vsphere guest_id, as recent versions of fog require it