Send environment vars to diskimage-builder per image

Some diskimage-builder elements need some environment vars to be
defined. Such as mirror, keys, etc... Add a env-vars section
into diskimages, so these vars can be set per different image
types to be using on diskimage-builder.

Change-Id: I472962d6425767d472f92bbfc1fc83befbb477fd
Merge "Send environment vars to diskimage-builder per image"