Certain environments use WARN for serious logging; however, it's very
possible to have machines without some of the fingerprinted keys
(public-ipv4 and public-hostname specifcally).  Setting log level to
INFO seems more consistent with this possibility.

Changed the keys map to reflect that.

This PR:

* Makes AWS network speeds more granular
* Makes `network_speed` an override and not a default
* Adds a default of 1000 MBits if no network link speed is detected.

Fixes #1985

Fix URL. It was printing an error message on startup:
```
2015/11/13 15:49:21 [ERR] fingerprint.env_aws: Error querying AWS Metadata URL, skipping
```

By the way is it safe to use latest? Is there a chance that Amazon decides to change the format of the metadata? It could be safer to use something like `http://169.254.169.254/2014-11-05/meta-data`

GCE and AWS both expose metadata servers, and GCE's 404 response
includes the URL in the content, which maatches the regex. So,
check the response code as well and if a 4xx code comes back,
take that to meanit's not AWS.

- move isAWS to env, use in other places
- fingerprint/network.go is no empty; removed