replace use of dhcp_lease_time with dhcp_lease_duration

replace use of dhcp_lease_time with dhcp_lease_duration

Having 2 separate dhcp lease period flags (dhcp_lease_duration
used by the quantum db plugin, and dhcp_lease_time used by the
dnsmasq dhcp agent) means that you could potentially have different
lease periods being set, and used by different parts of the codebase.

It seems to me there is no real need to have 2 separate flags, so this
commit basically replaces references to dhcp_lease_time (which is used
by the dnsmasq dhcpagent) with dhcp_lease_duration. This way, there
is only one flag to set, and both the quantum db plugin and dnsmasq
dhcp agent will use the same lease period.

Change-Id: Ia99dfa9284b6e737144f61a80c59cd898c0949bb
Fixes: bug #1192550