Add support for specifying probe headers.

Added support for probe headers, eg:

  headers = {
    'User-Agent' => 'copperegg'
  }

  copperegg_probe "#{tenant.name}-restapi" do
      provider 'copperegg_probe'
      action :update
      tags [env_tag, tenant.name]
      headers headers
      probe_desc description
      probe_dest uri
      type 'GET'
  end