Install OpenStack clients outside of venvs in boot-stack.

Trunk nova-client was being shadowed in the boot-stack element
by a nova-client installed via apt-get.

This change installs nova/glance/quantum clients outside of
venvs so that they take precedence.

Change-Id: Iddd7e2319d37723174b7251d095b5e8d61fad0f9