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
Repos should be read from the API so we won't miss any of them the next time we'll run the script.
See https://developer.github.com/v3/repos/#list-organization-repositories