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.
14 jobs for fg/dhcp_options2env-rh1663253 in 38 minutes and 13 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #366873
allowed to fail
checkpatch

00:01:00

manual #366878
allowed to fail manual
t_centos:7.5.1804
passed #366879
t_centos:7.6.1810

00:26:40

passed #366882
t_debian:9

00:18:49

passed #366884
t_debian:sid

00:19:27

manual #366883
allowed to fail manual
t_debian:testing
passed #366874
t_fedora:28

00:23:12

passed #366875
t_fedora:29

00:38:10

passed #366876
t_fedora:30

00:25:12

manual #366877
allowed to fail manual
t_fedora:rawhide
passed #366880
t_ubuntu:16.04

00:19:48

passed #366881
t_ubuntu:18.04

00:19:31

 
  External
passed NetworkManager test verification

passed NetworkManager test verification

05:27:53