This was marked as deprecated in commit 95dbd03a0bc4867bf8520539ed08f4068adf2f33.
That was in Havana. We have given enough time for admins
to upgrade to the new variable - dhcp_lease_duration.
UpgradeImpact
TrivialFix
Change-Id: Ie08c8f66b84cc577a99355f6977f946398b0a94a