Fix failure to complete pull-tools in tripleo gate

The required TRIPLEO_ROOT envar was not included in initial version
of this script, so it failed.

With this patch I've confirmed that the build now completes and shows
up as "ready" in nodepool image-list.

Change-Id: I3006b8c0d55317e1640149e92f47bc265030087e
Merge "Fix failure to complete pull-tools in tripleo gate"