Add tools/gate/build-images script
Add tools/gate/build-images script

It will simplify configuring and tuning for our upcoming images
gating. In gate job we'll just exec this script and it will do
all the things we need. Let's start just with the building all
images for the specified plugin.

Change-Id: Iecef82bec8eb8d10218c5ad2b763ccabb41053ad

uladz/sahara-image-elements,bigfootproject/sahara-image-elements,crobby/sahara-image-elements,openstack/sahara-image-elements,bigfootproject/sahara-image-elements,uladz/sahara-image-elements,openstack/sahara-image-elements,crobby/sahara-image-elements