checkEndpointSubsetFormat ensures that,
1. the current master's IP is in the list of addresses
2. the number of IPs in the list exactly matches the master count
This is problematic while masters are in the process of starting
because it causes frequent updates to the kubernetes endpoints until
all masters have started and added themselves to the list.
checkEndpointSubsetFormat should report success if the current
master's IP is found and the count of addresses is less than or equal
to the expected count.
These are "Bootstrap Controllers" as distinct from
the controllers in the controller-manager binary - they
are necessary for the cluster to start running.