Unify and document environment variables for debugging

There are a lot of environment variables used for debugging:

  • DEBUG
  • LEAP_DRYRUN
  • LEAP_PRIVATE_BRIDGE_CERT
  • LEAP_KCP
  • OPENVPN_VERBOSITY
  • SKIP_VERSION_CHECK
  • LEAP_PROVIDER
  • LEAP_PRIVATE_BRIDGE_CERT
  • MOTD_URL
  • LEAP_OPENVPN_EXTRA_CONFIG
  • LEAP_GW
  • UDP

They are not documentend in the README. Maybe we can remove some of them. Maybe combine with (Unify logging and add log level) #771 (closed)