vyatta-openvpn: add 'set interfaces openvpn vtun0 persistent-tunnel'

Add the option to make OpenVPN interfaces (TUN/TAP device) persistent
across client restarts.  This patch expands on the previous fix for
Bug #7 and Bug #313, where the interface description was maintained
for 'show interfaces openvpn', but ifAlias was unset and ifIndex
changed, following a client reset.

Bug #7 http://bugzilla.vyos.net/show_bug.cgi?id=7
Bug #313 http://bugzilla.vyos.net/show_bug.cgi?id=313