There won't be a Salt `0.20.x`.
Skip known state internal keys.
Properly handle requirements files not found.

If a requirements file is not found, the `pip` salt module does not even execute the `cmd` call, so, there's not `retcode`, `stdout` or `stderr`.
There won't be a Salt `0.19.0`.
Ignore known salt internal arguments.
Double to single quotes.
Add some more known state internal keywords.
Merge pull request #8237 from s0undt3ch/develop

Some minor fixes