running gnocchi in a virtualenv means ceilometer collection can't find gnocchi.ceilometer.dispatcher

If you run a devstack with gnocchi with USE_VENV=True then the ceilometer-collector cannot dispatch measurements to gnocchi:

    2015-06-08 17:12:31.443 15340 ERROR oslo_messaging.notify.dispatcher RuntimeError: No ceilometer.dispatcher extensions found

No surprise: the setup.cfg that adds gnocchi.ceilometer.dispatcher to entry points is in a different virtual env.

Possible fixes to this problem:

* Make gnocchi use ceilometer.venv for its venv. Instead of this in plugin.sh:

       PROJECT_VENV["gnocchi"]=${GNOCCHI_DIR}.venv

  do

         PROJECT_VENV["gnocchi"]=${CEILOMETER_DIR}.venv

* Accelerate moving the ceiolometer dispatcher code to ceilometer.

* Temporary workaround: don't USE_VENV

The first is not tested but is likely something we'll to make work across the ceilometer suite as we break up the repos but still want some kind of integration testing.