Support 'infinite' dhcp_lease_duration

Process a dhcp_lease_duration value of -1 as 'infinite'
when setting the dnsmasq dhcp-range values to support
cases where it is undesirable for instance dhcp leases
to expire.

DocImpact

Closes-Bug: #1315430
Change-Id: I1cc3cfebfec355014e6d5b5cbd656a1300c43c0b
Merge "Support 'infinite' dhcp_lease_duration"