Adding unit tests for zfs.exists and zfs.create [WIP]

### READY TO BE MERGED!!

@cachedout @rallytime 
I know. I have a couple if them fixed on as local branch which is far from finished.

Those test modules are wrong although they only fail if executed standalone.

If the documentation shows salt modules being imported before the ensure call, then it's also wrong.

The tests suite doesn't fail because it happens to have all required paths in python path, so it just works. Again, theybonly fail when run standalone.