Write failsafe hosts to file

[pederhan]

When the failsafe is hit, we need some way to inspect the hosts that are to be added/removed. This PR makes the application dump the hostnames to add and remove to a JSON file in a user-configuration location with the option zac.failsafe_file.

The JSON file has the following structure:

{
  "add": [
    "host1.example.com",
    "host2.example.com",
  ],
  "remove": [
    "host3.example.com",
    "host4.example.com"
  ]
}