Add DEP-8 / autopkgtest integration tests for system-image

[Barry Warsaw]

LP: #1321481 requests adding DEP-8 integration tests to system-image that must pass in order for dependencies of s-i to land. I think it also makes sense to add integration tests to u-d-m's autopkgtests so that we can have a greater assurance against regressions.

My first thought for both was to run the full system-image test suite. That might be overkill though, but a subset of tests could be crafted that will at least give a good smoke test of s-i and udm. Here's a thought:

# Set up the environment
tox --notest -r
# Run a subset of the tests, but which does touch udm.
.tox/py34/bin/python -m nose2 -P test_download -vv