Add centos7 image to nodepool d-i-b config

Change Ida38cba0a303ab77432d5d354f1952e00ebfa00e adds support to
tools/build-image.sh for building test-images with these elements and
release (note this has been moved into the separate config repo).
This adds support for nodepool to build the image and deploy on
hpcloud.

Note this retains the original centos7 nodes using the RAX provided
image, which are currently only used in an experimental devstack job.
These will be removed for this when stable.

Change-Id: Ic1848b98625603bf37c0e3dfaf1d0ef6f2c6ac74
Merge "Add centos7 image to nodepool d-i-b config"