Adds test-generators for valid and invalid configuration files

[funkyfuture]

this adds testing of valid and invalid configuralions that i wrote for github.com/docker/pull/1355

[mnowster]

Hi @funkyfuture,

I've reviewed these test cases and I'm not going to merge this PR. The format of the tests is not in a style/format in-keeping with the rest of the project and is not as explicit in what it is testing for each case. This makes it harder to read and less accessible.

A lot of the test cases that you report as "failing" are only failing according to newly-invented criteria, which (as stated in the discussion on PR docker#1808) is out of the scope for this first piece of work. For example, we don't currently validate domainname beyond requiring that it's a string.