In some larger setups, the individual IP of the management node for a

cluster may not be fixed. This makes it very annoying to set the allowed

IP whitelist for the healthcheck endpoint.
This change makes it so the DJBLETS_HEALTHCHECK_IPS setting is now

formally a comma-separated list which can include either individual IP

addresses or CIDR subnets.
This change also adds unit tests for the healthcheck endpoint, which

didn't have any.

Verified that both individual IP and subnet matching worked as

  expected.
Ran unit tests.