Adds the following subcommands to the agnhost binary:

- dns-server-list: outputs the host's DNS server list with which it was
  configured with. This can be found in /etc/resolv.conf on Linux and
  through some powershell commands on Windows.
- etc-hosts: outputs the host's hosts file. This can be found in /etc/hosts
  for Linux, and C:/Windows/System32/drivers/etc/hosts for Windows.
- pause: pauses the binary's execution. This can be used to keep the Pod in
  a Running state for various reasons, including executing additional agnhost
  commands.

Refactors bits of the code to avoid duplication.

Adds a README for the agnhost image.