Fix dhcp_release lease race condition

There is a possible race condition when delete or updating fixed_ips
on ports where an instance could renew its ip address again after
dhcp_release has already been executed. To fix this, the order of
reload_allocation and release_lease need to be switched. This way an
instance will not be able to renew it's ip address after it is
removed from the host file.

Fixes bug: 1237028

Change-Id: If05ec2be507378c634f5c1856dab0fbd396f43cc
Merge "Fix dhcp_release lease race condition" into milestone-proposed