Allow to specify a custom network server hostname

[smortex]

When sending metrics to multiple services on distinct ports on the same
node, in order to avoid conflicts we need to allow overriding the server
hostname on a per resource basis.

Example use case:

class { 'collectd::plugin::network':
  servers => {
    'collectd' => {
      hostname => 'remote.example.com',
      port     => 25826,
    },
    'influxdb' => {
      hostname => 'remote.example.com',
      port     => 25827,
    },
  },
}