Allow input def without options defined

[jbiel]

Some of the telegraf inputs don't have many options and have good defaults (haproxy for example.) In the case of these type of modules, it'd be nice to be able to specify an input with just a plugin_type. If options isn't passed the input template fails to apply when it tries to iterate over the empty options hash.

[yankcrime]

I'll look into this, but you don't have to use the dedicated telegraf::input defined type, you could just do something like:

class { '::telegraf':
  inputs => {
    'haproxy' => {},
  },
}

And it will work, e.g:

#
# INPUTS:
#
[[inputs.haproxy]]