(PUP-2909) enable static checking with rubocop 

This enables rubocop with a few selected checkers (cops). These selected allowed finding a few errors which are also fixed along with this checkin. The enabled verifiers include:

Lint/UnreachableCode
Lint/ConditionPosition
Lint/UselessComparision
Lint/LiteralInterpolation

These style checkers were found to be triggered on a very small number of problems which were found to be real problems. I have also marked a few checkers as 'MAYBE' which I feel might help puppet development, but as of now results in a large number of warnings. The rest of the checkers should only be enabled after verifying that they are indeed useful for puppet codebase.
I agree with @Sharpie that adding this check would probably be good, but let's keep this a little more focussed for now.

@vrthra, I think this discussion about which of these robucop checks should be turned on would be better done on the puppet-dev mailing list.