vyatta-openvpn: add 'set interfaces openvpn vtun0 keep-alive'

Add the option to set the parameters passed to the openvpn keepalive
helper.  This option has two configuration nodes, interval, for
specifying the interval between keepalive packets, and failure-count,
used to calculate (interval * failure-count) the maximum time allow
before a restart is triggered.

http://bugzilla.vyos.net/show_bug.cgi?id=360