Integrate pylint into Travis build process

So, I'm finally starting to tire of making lots of pull requests to fix the relatively easy/simple/obvious issues that pylint reports. Hence, this pull request :stuck_out_tongue_winking_eye:

This pull request adds a pylint step to Salt's Travis build. The configuration has been painstakingly tweaked to avoid checks that are prone to false positives or with which the codebase does not already almost completely comply.

This PR also adds a select few pragma comments to manually ignore outstanding instances of a few high-value checks (it's several commits for ease of future auditing, but it adds up to only 14 Python lines changed in total).

With this configuration, pylint currently reports [only 5 violations](https://travis-ci.org/cvrebert/salt/jobs/8983127); these are due exclusively to issues #4681 and #5865.