Activemq usage params

This adds management of the activemq.xml memory tuning parameters documented at:
http://docs.puppetlabs.com/mcollective/deploy/middleware/activemq.html#memory-and-temp-usage-for-messages-systemusage
to the template, and exposes them as class parameters using the current default values hard-coded in the template.

I've been hitting memory issues as documented at the above link, but haven't been able to fix them as it would require either manual changes to the template, or using an external config string or template.

I've stuck with the `<%= scope["mcollective::activemq_tempUsage"] %>` convention as that's what the rest of the template uses (rather than using local variables where the template function is called).