[ACS4.4]Cherry-pick CLOUDSTACK-6859

[Min Chen]

Hi Daan,

Would you please cherry-pick the following commit from 4.4-forward to 4.4?

Commit: 961cd7657e7dbb69d497123071db3156bd15d8e9
CLOUDSTACK-6859:Management Server PermGen run out of memory after some time due 
to class leak.

Thanks
-min