Fix ParentAwareNamingStrategy to set ObjectName properties for the
'identity' and 'context' attributes. Also update JmxAutoConfiguration
to ensure that the ParentAwareNamingStrategy is created in each context
and that the `mbeanExporter` bean is created. Prior to this commit the
nested @EnableMBeanExport class always meant that the mbeanExporter
condition never matched.

Fixes gh-2243