Fixes broken panel registration for source install method
due to python 3 changes changing the horizon_lib_dir path
dynamically. This sets changes the horizon_dashboard_panel_dir
var to utilize the horizon_lib_dir fact being set in in the
horizon_install_source.yml playbook.
Closes-Bug: 1848561
Depends-On: https://review.opendev.org/689323
Change-Id: Ide5181c14fdc2c67df4df3adaa155d5959b4c468