Pass intended TRIPLEO_ROOT to setup-env

Change-Id: Ifbd2f098757c0514f225df640bcd894a8c73d1fa
Element to build and maintain local pypi mirror

We use the instructions from diskimage-builder's pypi element README to
create a copy for use with elements which need to build images more
rapidly.

Change-Id: I452c35f1b33a3d6c8118bf762b1a58be62cfe8ea