Delayed the OqParam validation

This fix is needed to make the check in `check_hazard_risk_consistency` work. It solves this problem reported by Catalina: https://bugs.launchpad.net/oq-engine/+bug/1393826
It requires a companion PR on commonlib: https://github.com/gem/oq-risklib/pull/84
The tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/863