Currently, upgrading a horizon package/venv does not explicitly trigger
a restart. A restart will only occur if an explicit or implicit config
change occurs.
This commit adds several restart notifiers such that a horizon package
upgrade or venv replacement will force a restart.
Note that this issue exists in all OpenStack roles at the moment, but
the remaining roles are being addressed in their re-deploy venv
reviews. The horizon re-deploy venv change was merged a while back
which is why we need the separate review for this.
Change-Id: I890e46bb7a2973c3582be7a37ee70730d1e3ffef
2c18720487