use better mechanism for enums in gs_utils.py

This will allow more flexible subclassing of the GSUtils class: subclasses will automatically inherit the various constants.

That more flexible subclassing will allow us to easily wrap common's gs_utils.py with an extended version within the buildbot repo, as discussed in https://codereview.chromium.org/405653004/diff/20001/slave/skia_slave_scripts/build_step.py#newcode60

R=borenet@google.com

Review URL: https://codereview.chromium.org/405083005