ActiveSessionsLoadMetric throws AttributeNotFoundException when <distributable/> webapp is deployed

The model mbean registered with the standard session manager contains an "activeSessions" mbean attribute.
The standard mbean registered with the clustered session manager contains an "ActiveSessions" mbean attribute.

If both non-distributable and distributable webapps are deployed (this is the case if a <distributable/> webapp is deployed using the "all" profile), the ActiveSessionsLoadMetric will throw an AttributeNotFoundException when trying to fetch the active sessions using an invalid attribute name.