This requires three main changes:
* setting CEILOMETER_BIN_DIR appropriately
* running the various services with a full path
* explicitly installing optional python modules (for mongo and
virt drivers, if configured) during the install phase
In the process of making this work it was discovered that setting
CEILOMETER_BACKEND to something like 'foo' would cause the backend
to be configured to use mongodb but for the mongodb system packages
and related python modules to not be installed. Fixing this was used
to validate the install process under USE_VENV.
Change-Id: I35fbfa76bdd60a22ba90b13666b06eeb961dddb3
027e2ea741