Glance/Cinder/Keystone/Swift: don't hard code venv

Updates the cinder/glance/keystone/swift 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: Ic2cbbf48bad0db092fe6c0a5a9ddda6f883a499b