Add package install support for tuskar-ui

Support installing from packages and source for the tuskar-ui element.
The post-configure.d script is also updated to only use the virtualenv
for Horizon if the $HORIZON_VENV_DIR environment variable is defined.

Change-Id: If42ca22fb173df0b35155ce760b94c1c6f2f9d28
Merge "Add package install support for tuskar-ui"