Allow configuring Statsd host and port from env vars

[zorbash]

This PR changes exometer_report_statsd to be configurable from environmental variables.

It can be configured as follows (Elixir code):

config :exometer,
  report: [
    reporters: [
      {:exometer_report_statsd, [hostname: {:system, "STATSD_HOST"}, port: {:system, "STATSD_PORT"}]}
    ]
  ]

The goal is to build the release artefact using a tool like Distillery just once for multiple environments (staging / production) and supply all the required configuration via env vars.

If this behaviour is decided to be merged, I can also work on the other reporters.