Reintroduce package openvswitch-ipsec

• Use upstream ovs-monitor-ipsec.
• Patch for Python 3 and strongSwan configuration changes.

Build checked by local sbuild on sid.

Tested using tutorial provided by upstream (GRE/PSK only): http://docs.openvswitch.org/en/latest/tutorials/ipsec/

Known issues:

• Geneve tunnel with IPsec does not work, similar behavior as in https://mail.openvswitch.org/pipermail/ovs-discuss/2019-September/049240.html)