Automatic discovery of routes-to-push doesn't work on GCE.

[merlijn-sebrechts]

Automatic discovery of routes-to-push doesn't work on GCE because each vps thinks its subnet mask is 255.255.255.255

[merlijn-sebrechts]

You can workaround this by adding a static route on the VPN server host. This should be documented in the README of the charm.