Adding a test function which allows us to use elements to perform
element-specific tests. In order for this to work sanely, also adding
some configuration to our break system so we can assert on negative
tests.
Also adding a test for apt-sources to verify this code actually works.
Change-Id: I378a74255010eca192f5766b653f8a42404be5ea
Set the pbr option 'warnerrors' to make build_sphinx turns warnings into
error. Fix all warnings.
`tox -edocs` will thus abort whenever someone introduce a new error.
Change-Id: Id6d09768a241866e1fdc1a1e2bf90336f5c5087d
