Allow the log threshold to be configured for managed JBoss AS 6

The managed JBoss AS 6 adapter does not explicitly set the log threshold on the server controller. As a result, the server controller defaults the log threshold to DEBUG. You can see this setting reflected in the start command:

-Djboss.server.log.threshold=DEBUG

This threshold produces an enormous amount of log statements, which get redirected to the server/default/log/output.log file.

Here's a comparison of the log file size between various threshold levels:

DEBUG: 2.2 MB
INFO: 80 KB
ERROR: 76 KB

The server starts much faster when the threshold is higher, so we should allow this setting to be configured. If a configuration value is not set, the level should default to INFO (rather than the fallback of DEBUG).