Support starting the maven build from a directory distinct from rhq root

Why: because maven's parent shell's pwd is not always under our control. I have seen this happening in Jenkins or when I build using a shell script started as Eclipse's external tool.
Yes, as already mentioned on IRC, this is a build script, for which only the java used to run maven is important. @burmanm said this is OK, as the RHQ source level is probably at Java 7 anyway. You mean that @burmanm is not right? 

It is true that RHQ target level is at Java 6, but that is fully reconcilable with this build script using Java 7.