Fix opt helpstring for dhcp_lease_duration

Recently (https://review.openstack.org/#/c/92453), the
functionality of dhcp_lease_duration was changed to support
infinite lease times.

However, this change failed to update the associated help text
with the option declaration. This patch simply updates the text
so that the new functionality can be understood by the user.

Change-Id: Id9dbe11599c55b447387df4a2d6639b7e8dba6ce
Merge "Fix opt helpstring for dhcp_lease_duration"