Allow to override stats metrics host

[LotharSee]

For now, there is no way to change the host of metrics reported thought statsd.

This PR adds an optional field host to to DogStatsApi methods which allow to override this value.
Basically, it implements the magic tag host: DataDog/dd-agent#1164

[LeoCavaille]

Cool!
Could you see what's going on with the CI build? Seems like one test failed.

[LotharSee]

@LeoCavaille good catch, it was a sneaky non-deterministic test, thanks now it's fixed!

[LotharSee]

Also, there is currently no way with dogstatsd to define the host attribute of a metric.
Two suggestions (which in fact concerns https://github.com/DataDog/dd-agent/)

• Define a new syntax (like |host:foo)
• (Better) Consider that a tag host:foo will erase the host attribute with foo. It also avoid the strange situation where the host attribute and the tag host differ.

[clutchski]

I like the idea of a tag called host. If you submit host:foo, it will override. If you submit host: it can be a shorthand to not patch the hostname at all (and not the host tags either, which would reduce contexts for a few cases)