add NANOBOX envr for application switches

[ndac-todoroki]

Currently you can not know if the application is run within a nanobox or not, by the environmental values.
This PR adds an environmental value names NANOBOX so the applications can, for example, recognize this and switch settings with it.

e.g.

# inside config/dev.exs
if System.get_env("NANOBOX") do
  IO.puts "*** Overwriting with Nanobox-specific settings."
  import_config "dev.nanobox.exs"
end