Fix failover of az-specific loadbalancers
Fix failover of az-specific loadbalancers

There is a bug exists in CalculateAmphoraDelta class
If there is no such a key as constants.MANAGEMENT_NETWORK
in availability_zone dictionary, method get will return None
It means that management_nets list will become [None], not
CONF.controller_worker.amp_boot_network_list as it supposes to be
It breaks the following failover flow

Story: 2008669
Task: 41957
Change-Id: I435c40a5493828bce24c2850182f0ddc8e5bdd1f

openstack/octavia,openstack/octavia,openstack/octavia