Use information from the dnsmasq hosts file to call dhcp_release

Certain situations can cause the DHCP agent's local cache to get out
of sync with the leases held internally by dnsmasq.  This method of
detecting when to call dhcp_release is idempotent and not dependent on
the cache.  It is more robust.

Change-Id: I4eafd9cfb94a77a2f0229f89de5483dad23725cf
Closes-Bug: #1250644
Remove release_lease from the DHCP driver interface

Neither the midonet nor the linux dhcp driver needs a call to
release_lease.  Removing this from the driver API simplifies more code
in the DHCP agent.

Change-Id: Ib144b2bf3720b1b999205ace1a7d1ffe5ef3b167
Closes-Bug: #1269505
Merge "Use information from the dnsmasq hosts file to call dhcp_release"
Merge "Remove release_lease from the DHCP driver interface"