detail discussion is here
Summary of discussion thread:
Karaf minimal distribution has issue with feature:install webconsole that traces down to a feature request to add admin support for child instances on the minimal dist.
Fixed on karaf-2.2.x: http://svn.apache.org/viewvc?view=revision&revision=1422965
Fixed on karaf-2.3.x: http://svn.apache.org/viewvc?view=revision&revision=1422978