ovs-monitor-ipsec: Add ability to traverse NATs

ovs-monitor-ipsec: Add ability to traverse NATs

Stable versions of ipsec-tools have a bug that prevents our using
transport mode through a NAT box.  Even though the bug has been fixed
for years, it is only available in the 0.8 alpha release of ipsec-tools.
This commit modifies our configuration to allow NAT traversal with that
version of ipsec-tools.

NB: An official package for this version of ipsec-tools is not yet
available on Debian, so we're requiring a custom version number.