Add action-managed-update flag to limit changes on routing

[Alvaro Uria]

When "juju config" changes any value, routes will automatically trigger a "remove_routes" (the current live routes) and "apply" the new routes. However, there are customers that want to limit changes during approved operations or pre-agreed weekly maintenance windows.

A new config parameter "action-managed-update" should exist (default: True) to skip live changes except if an action is run. Said action would do the same process as the current one: remove current routes and rules, apply the new ones listed in the config parameter.

New rules and routes should be validated before removing the live ones. In case there is something wrong, the action would fail and the "old" routes would continue working.