JAAS configuration should delegate unknown application names to pre-existing configuration

As reported here: https://issues.cloudera.org/browse/DISTRO-66 it is impossible to use secured Hadoop inside an application that relies on other JAAS configurations. This is because the static initializer of UserGroupInformation replaces the JAAS configuration, but we don't delegate unknown applications up to whatever Configuration was installed previously. The delegation technique seems to be used by JBoss's XMLLoginConfigImpl for example.