Dogstatsd magic tags

Currently, If someone report a `host` or `device` tag with DogStatsD, the tag will conflict with the `host` and `device` attribute in the webapp. To avoid this confusion (and to allow not to set these optional fields with dogstatsd), this PR brings:

* `host:foo` and `device:bar` tags will override the attributes `hostname:foo` and `device_name:bar`
* if set to empty (`host:` or `device:`), these fields are removed. Useful to remove the cardinality of metrics when they aren't useful.
* a PR is already opened on dogapi to implement it https://github.com/DataDog/dogapi/pull/116