T778: T782: dhcp-server: XML and Python rewrite

This commit changes in addtion the DHCP server config syntax as defined
in "T782: Cleanup dhcp-server configuration".

Replace boolean parameter from the folowing nodes and make it valueless.
This requires a migration script which is tracked with this task

• set service dhcp-server shared-network-name <xyz> subnet 172.31.0.0/24 ip-forwarding enable (true|false)
• set service dhcp-server shared-network-name <xyz> authoritative (true|false)
• set service dhcp-server disabled (true|false)
• set service dhcp-server dynamic-dns-update enable (true|fals)
• set service dhcp-server hostfile-update (enable|disable)

Replace the nested start/stop ip address from "subnet 172.31.0.0/24 start
172.31.0.101 stop 172.31.0.149" to "subnet 172.31.0.0/24 range <foo> start" and
"subnet 172.31.0.0/24 range <foo> stop" where foo can be any character or number.

In addition the vyatta-cfg-dhcp-server package used it's own init/config file
for service startup. This has been migrated to the vanilla Debian files.

Copy 'on-dhcp-event.sh' from vyatta-cfg-shcp-server package commit 4749e648bca6.