dispatcher: add MS Azure endpoint env var expected by cloud-init

Some linux distros (e.g., RHEL, CentOS and Fedora) ship a dispatcher
script with their dhclient package in order to run user dhclient hook
scripts also when connections are run by NetworkManager.
The scripts convert the var names in the environment to the ones
expected in the dhclient hook scripts.
This feature is currently use by cloud-init to retrieve the MS Azure
server endpoint address, which is sent in the private dhcp option 245.
We just redefined the default dhclient var names for the private options
from "unknown_xyz" to "private_xyz". In order to let current cloud-init
version to be able to retrieve the Azure server endpoint address, add
the legacy var name "unknown_245" to the dispatcher script environment.