[METRICS-390] Add global host tags and add tag writing from metadata using target_set 

1) A global HostTags is added to write_tsdb that apply to all values sent from this instance of collectd.

2) Tagging and prefixing:
See https://collectd.org/wiki/index.php/Chains

See https://github.com/llnw/collectd/pull/8 for the target_set tagging implementation.

The global metric name prefix is removed and can be set using a match on Plugin name, including a wildcard.

The first rule simply prepends 'sys' to the all metric names but can be overridden later in the chain.  The second rule uses TypeInstance to pass in a value, and target_set strips it and sets a TSDB tag instead.
```
<Plugin write_tsdb>
  <Node>
    Host "localhost"
    Port "4242"
    HostTags "status=production deviceclass=www"
  </Node>
</Plugin>

<Chain "PreCache">
  <Rule "add_default_sys_prefix">
    <Match "regex">
      Plugin "*"
    </Match>
    <Target "set">
      TSDBPrefix "sys"
    </Target>
  </Rule>
  <Rule "tag_port">
    <Match "regex">
      Plugin "^someperlplugin$"
      TypeInstance "^8080$"
    </Match>
    <Target "set">
      TypeInstance ""
      TSDBPrefix "app"
      TSDBTags "port=8080"
    </Target>
  </Rule>
</Chain>
```