Address Spotbugs errors

[xsalefter]

1. Enable check.fail-spotbugs in each module (do not in root killbill project, otherwise, it wont build

2. Find a way to fix the error

3. If (2) is either:

   • too costly
   • almost impossible to fix
   • it is as is as designed/intended

   then exclude it to <killbill-root-project>/spotbugs-exclude.xml file.

4. Remove the @SuppressFBWarnings annotations and use spotbugs-excude.xml file instead.

[xsalefter]

Fix order:

killbill-api
killbill-util
killbill-tenant       
killbill-account      
killbill-catalog      
killbill-currency     
killbill-subscription 
killbill-entitlement  
killbill-junction     
killbill-usage        
killbill-invoice      
killbill-overdue      
killbill-payment      
killbill-beatrix      
killbill-jaxrs        
killbill-profiles         
killbill-profiles-killbill 
killbill-profiles-killpay