Filter disk, io & network metrics by device

[clutchski]

This pull request allows us to configure a device_blacklist_re in datadog.conf. This will be checked against all disk & io devices so we can easily filter out fake devices.

This pull request contains a few things:

• device filtering on disk metrics
• device filtering on io metrics
• device filtering on network metrics
• run unit disk metric parsing tests on all platforms (since the parsing code is not os specific)
• removes a bunch of base exceptions
• random little cleanups
• added a platform library for cleaner platform detection

@remh @elijahandrews @alq666 can you review?

[conorbranagan]

Should we move this to a checks.d as we did with the network metrics so we can configure the blacklist in there instead of continuing to crowd datadog.conf? Then we could add a list of specific devices to skip and/or multiple regex expressions.

[elijahandrews]

Is it worth noting that this file will be overwritten when the Agent is updated?

On a related note, should we consider making this file a 'configuration file' as far as packaging is concerned? Here is how it would be handled in a Debian package.

[clutchski]

I think the right solution is to make system checks have default behaviour without yaml and optionally allow people to override.

This will:

• prevent us from blocking installers with questions like "do you want to overwrite this config?"
• let system checks run without manual work for new users
• allows optional config

Thoughts?

@conorbranagan @elijahandrews @alq666

[elijahandrews]

As discussed this morning, this is what I think we should do: #620