Fix TRACKING_DEPENDS mode

TRACKING_DEPENDS mode was added in I89677fd54635e82b10ab674ddeb9ffb3f1a755f0

TRACKING_DEPENDS creates a global virtualenv and assumes it exists for
all pip_installs (except for installing virtualenv), so we need to set
this up before any other pip install

Change-Id: Iebe6295913094b32124295e7c5e674e1cebaaa87
First pass at GLOBAL_VENV

Add option to use a global virtualenv so we can have two copies of
dependencies installed on one system (for upgrades)

Change-Id: I172df20cbce600aebced1132e173453c0464caf6