Switch TRIPLEO_ROOT

devtest-gate git clones the correct revision of each repository to
/opt/stack/new, once the tripleo repositories are included in this list
https://review.openstack.org/#/c/68645/
we can use this as TRIPLEO_ROOT without having to worry about setting
the correct revisions ourselves.

Change-Id: I5d92dc2e66065d0d2aa3f6d48100d178ace43cd0
Merge "Switch TRIPLEO_ROOT"