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

### READY TO BE MERGED!!

@cachedout @rallytime 
@s0undt3ch I just followed the documentation on creating unit tests and also looked at the unit tests module at https://github.com/saltstack/salt/blob/develop/tests/unit/modules/linux_sysctl_test.py in which salt modules are imported before the ensure in syspath call. This is currently being done in many other test modules as well