When specifying failure-count in the OpenVPN interface configuration, the user specifies how many lost pings we want to tolerate.
However, the OpenVPN daemon expects a parameter expressed in seconds. This means that the renderer should convert the count to seconds, but this does not happen and the wrong value lands in the configuration.

Fix this issue by performing the right computation during config rendering.

It's not truly a syntax change, but rather a semantic change which needs a small migration.