Remove DHCP lease logic

Previously neutron was keeping track of dhcp lease time in order
to ensure it didn't hand out an ip address that was already leased.
This patch removes that logic and instead leverages the dhcp_release
utility. This allows us to reuse ip addresses immediately after a port
is deleted. This patch also bumps the lease time to 24 hours instead
of 2 minutes with reduces the amount of dhcp traffic.

DocImpact

There is a DocImpact for this bug related to the upgrade path. One should
first upgrade their dhcp-agents. Then wait till the dhcp_lease time has
expired. Lastly, update neutron-server in order to avoid the case where
an instance is deleted and the dnsmasq process has not released the lease
and neturon allocates that ip to a new port.

Fixes bug: 1202392
Implements blueprint: remove-dhcp-lease

Change-Id: Ifcb4f093c92904ceb896438987d53e692eb7fb26
Merge "Remove DHCP lease logic"