Bugfig, allowing all paths to have spaces

[AndrewFarley]

Fixes #242

Problem

Could not use this plugin with any path that contains a space character due to missing/insufficient shell escaping/quoting.

Solution

I'm sorry I couldn't find a simple "quote-all" solution similar to what was in place pre-merge. These solutions fall apart because certain things you can't quote or docker freaks out, for example features like the 'slim' would fall apart... eg: (see the last line)

   [ '-m',
     'pip',
     'install',
     '-t',
     '\'/root/test me/serverless-python-requirements-farley/tests/base/.serverless/requirements\'',
     '-r',
     '\'/root/test me/serverless-python-requirements-farley/tests/base/.serverless/requirements/requirements.txt\'',
     '" && find /root/test me/serverless-python-requirements-farley/tests/base/.serverless/requirements -name \\"*.so\\" -exec strip {} \';\'"' ]

The "simple" single test you added for testing in a subfolder only tests one of many use-cases that use various inputs for volumes, caching, etc. This MR I tested (as you see above) running ALL tests from a folder with a space in it, confirmed working. Just, not as clean as I would like, but I spent 4 hours trying to make it work with a quote all args type mechanism. If you don't like this MR's style, please let me know, but I'm out of effort/time/etc. I can post my diff for my attempts as using a simpler quote-all if interested. I did not, however, go test if this breaks anything on Windows. :\ We really need automated testing on Windows somehow @dschep.

[AndrewFarley]

Ah crap, idk what is up with that failing test, but I'll look into it later this evening. :P

[dschep]

The error was docker not found which I assumed was a CircleCI issue, and upon retrying, tests pass, so it was!