[bug] Checksum cache is not invalidated on VPN server change

[nemesifier]

If a VPN server is changed, there could be some variables (eg: hostname), which may be used in the VPN-client templates and would need to invalidate the cache of all the devices using those templates, but this is not being done now.

The result is that the config checksum is outdated and devices do not update their config.

To fix this we shall do the following:

• when a VPN server is changed, launch a celery background task
• the task shall call a method on the VPN server model which triggers invalidation
• the method shall look for any vpn client template related to the VPN server, then find all the devices which have the template and invalidate the cache for them