add a script to synchronize labels on all OCA repositories

Here is a script to help definition of labels.

It was already run but on l10n-belgium where I have no access rights.

Here is the talk about labels: https://github.com/OCA/maintainers-tools/issues/29
An entry point should be added in https://github.com/OCA/maintainers-tools/blob/master/setup.cfg#L20