[BZ 1070668] Incorrect default configuration of RHQDS resource

Installer code fixed to properly fill xa-datasource-class property when
creating RHQDS. Class names for postgres, oracle live as constants in java
class.

This is quite easy fix, the only questionable thing is if we'd like to expose xa-datasource-class name as property in rhq-server.properties or not (I vote for no). This property (or 2, one for pg, one for ora) were exposed in pre-AS7-based RHQ.