Add ability to use environment variables for image names

Environment variables:
* ubuntu_[vanilla|spark]_hadoop_[1|2]_image_name
* fedora_vanilla_hadoop_[1|2]_image_name
* centos_[vanilla|hdp]_[hadoop_1|hadoop_2|plain]_image_name

Change-Id: I2440be46ec28f7790fcb7d643ee0c15f11d2d8f4