jmeter: make username/password configurable
jmeter: make username/password configurable

Signed-off-by: Pierre-Alexandre Meyer <ff019a5748a52b5641624af88a54a2f0e46a9fb5@mouraf.org>

NeilW/killbill-integration-tests,NeilW/killbill-integration-tests,killbill/killbill-integration-tests