Allow pull requests to configure some parameters in scripts/hudson/narayana.sh

If the pull description has something like this:

export NARAYANA_BUILD=1 AS_BUILD=1 NARAYANA_TESTS=1 TXF_TESTS=1 XTS_TESTS=1 XTS_AS_TESTS=1 txbridge=1 QA_TESTS=1 [SUN_ORB=0] [JAC_ORB=1] [QA_TARGET=ci-jts-tests]

We could use it in scripts/hudson/narayana.sh if we detect it is a pull build. We might need to white list users we apply this to.