User AS client VPN expired certificate

[juagargi]

Both the VPN server and the VPN client have certificates that can expire.
Ensure, when reconfiguring a user AS, that the VPN connection is functional, including not expired certificates for the server and client VPN.

Otherwise the user gets the feeling that everything was well configured, but the VPN tunnel won't work. The user has no possibility of fix this from scionlab.org, as we require an admin to remove the VPN client object from the DB for this particular VPN connection.

Maybe related to #323