Ceilo/Ironic/Tuskar: don't hard code venv dir

Updates the ceilometer/ironic/tuskar elements so that we use a new
{SERVICE}_VENV_DIR variable (set via an environment.d hook)
to configure the virtualenv directory.

This is cleaner in that we aren't hard coding /opt/stack/venv
everywhere and also supports both isolated and shared
venvs (which some people would like to use).

Change-Id: I5a4deef3ed79f4b633f273a37cec2aa5a8dbc698