Horizon: dynamically set config time env vars

Updates the Horizon element so that we inject the
HORIZON_VENV_DIR environment variable dynamically at build time
into the os-refresh-config/environment.d directory.
This ensures that the correct value from the build time gets
used during configuration and fixes an issue where the
wrong local_settings.py file is "templated" when using
alternate values for HORIZON_VENV_DIR.

Change-Id: I00c0cb0fa03d6cce5a4625f362cc246d1461c576
Add a new common-venv element

If used will set two environment variables to allow a common
virtual environment to be used with --system-site-packages
enabled so that Nova can pull in python-libvirt.

Change-Id: I8151e2fb9aff322dbe2ffc5982f341120dba6c6c
Merge "Horizon: dynamically set config time env vars"
Merge "Add a new common-venv element"