Additional parameters to devtestrc

OVERCLOUD_COMPUTESCALE - number of compute nodes
TRIPLEO_CLEANUP - clean up VMs from previous devtest runs

Change-Id: I5c623734a69b2a6ea32ad2c96a59ed3d3afec637
Allow running Devtest as root

And also recommend it in the readme. There is a bug currently which
prevents running Devtest successfully as non-root user.

https://bugs.launchpad.net/tripleo/+bug/1278861

Change-Id: I990bd243f6f0af6251ffab861cb2425f750e2af7