Change in vdsm[master]: properties: Use python 3 compatible metaclass

gerrit-hooks has posted comments on this change.

Change subject: properties: Use python 3 compatible metaclass
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Patch Set 3:

* Update tracker: IGNORE, no Bug-Url found
* Check Bug-Url::WARN, no bug url found, make sure header matches 'Bug-Url: ' and is a valid url.
* Check merged to previous::IGNORE,         Not in stable branch (['ovirt-3.6', 'ovirt-4.0'])