Remove the need to use git-stoq

Move the environment setup from git-stoq to tools/env.sh. One can now start
working on Stoq directly by creating a checkout tree (e.g. using repo init
directly) and enter the sub shell by calling env.sh

It can be called by both:

  cd <checkout dir>
  stoq/tools/env.sh

  .. or ..

  cd <checkout dir>
  source stoq/tools/env.sh

This removes the need to use git-stoq for those that doesn't want to.

Change-Id: I1fc5e4aaf715786886ce8371e7cd587ac7023016