Add unit tests for version script

Add unit tests for version script

Adds unit tests for the version script to ensure it works in the use
cases OpenDaylight requires the script for. Unit Tests run against
Python 3 as we are only supporting Python 3 for lftools at the moment.

- Fix version script not running 'release' command
- Add pytest
- Test version script for 3 use cases:
    1) Version bump for release
    2) Version bump by x.y.(z+1) post release
    3) Version bump by x.(y+1).z

Change-Id: Ib36d9a84e2769863083400c434e1539b302869a8
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>