Migrate rich:tree in nuxeo-platform-shibboleth-groups-web

The JSF2 migration tool reported that rich:tree needs to be updated in:

• shibboleth_user_tree_widget_template.xhtml

According to http://doc.nuxeo.com/x/3Q45AQ:

• The var attribute should be moved from tag rich:treeModelRecursiveAdaptor to the upper tag rich:tree.
• The roots attribute should resolve to a list (a single element will not be taken into account anymore, silently)