Revert "remove the openvpn restart checking, doubting it is needed"

It is needed, at least until an openvpn server restart doesn't kill all
the clients.  That behavior is apparently due to the client openvpn
having dropped its privileges and being unable to accomodate interface
changes (i.e. ipaddr reassignments, etc).  This is discussed here:

  http://community.openvpn.net/openvpn/ticket/203

The cronjob allows nodes to recover at least a vpn tunnel after a server
bounce, which, even if olsrd doesn't deal with it, can allow connection
from the server for further recovery.

This reverts commit 676750c85db1878b4d30904e31adc819bfb95622.