docker: Support Stats on Windows

[endocrimes]

This PR adds support for fetching container stats on Windows.

This is based on the implementation in the Docker integration tests that
are referenced inline.

fixes #2897

[notnoop]

Should this be conditioning over the container OSType instead of the nomad binary OS? How do Linux Containers behave on Windows, do they report "normal" Linux stats similar to how Docker for Mac works?

[endocrimes]

@notnoop Potentially - But we don't currently officially support Linux Containers on Windows (they didn't work in many cases on <=0.8 anyway). It's easy enough to switch this out based on that if we want to support them though.

[notnoop]

lgtm.

Given that the code isn't actually platform-dependent, you may be able to avoid compiler flags and run tests. stats_test.go doesn't actually get any data from Docker so conditioning on test host OS isn't so great.

[endocrimes]

@notnoop gonna merge this and follow up with removing build flags later

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