Add support for the DD_TAGS environment variable

[oskarpearson]

Datadog agent uses a standard environment variable DD_TAGS which you can use to
set custom tags like the environment you're running in, the class of host,
and similar.

Prior to this version, the user could set only two tags: MY_POD_NAME
and STATSD_GROUPING.

This commit changes things so that:

• Stats sent now include tags set in the DD_TAGS environment variable,
  while keeping backwards compatibility with the MY_POD_NAME and STATSD_GROUPING
  environment variables.
• The 'tags' method isn't run once for every metric sent, since the result never
  changed from run-to-run. We now use a 'environment_variable_tags' method instead.
• The 'environment_variable_tags' method returns a string, rather than a hash.
  While datadog metrics are normally key/value pairs, you can create so-called
  'simple tags' which are just a string. For example, you may have a tag called
  "canary", and if it exists then the stats are from a canary instance.

Additional changes:

• Add section to the documentation on usage of DD_TAGS

[yob]

Lovely, thanks!

The kube-specific ENV vars always bothered me a bit, using a DD specific ENV var for a DD specific feature is much nicer.