vyatta-op-dhcp-server: Bugs #121 and #160

 vyatta-op-dhcp-server: update dhcp lease parsing logic based on type

Update the parsing logic for DHCP leases, so it allows certain lease
entries to be missing in /config/dhcpd.leases for backup and free
leases, but ensuring that they are present in active ones.

Bug #121 http://bugzilla.vyos.net/show_bug.cgi?id=121