Make the ComputedGroupsServiceImpl#allowGroupOverride() method configurable

ComputedGroupsServiceImpl#allowGroupOverride() only returns allowOverride which is initialized to true.
There is no way to change/override this beahvior.
It could be configurable through an extension point.

Moreover, in nuxeo-services/nuxeo-platform-usermanager-core/src/test/resources/computedgroups-contrib.xml we have this contribution:

<groupComputer name="dummy">
  <allowOverride>true</allowOverride>
  ...
</groupComputer>

But it seems like the <allowOverride> element is not taken into account in the groupComputer descriptor.