Add extension point to allow plugins to validate group creation

By implementing this extension point plugins can validate the creation
of new groups based on input arguments. E.g. a plugin could enforce
a certain name scheme for the group names.

To match the extension point for validating project creations a
CreateGroupArgs class was added that contains the arguments for the
group creation.

Change-Id: Id5a5909028d57ce588ed8b17d9435958d37668c1
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>