[Tests] Minor fixes in run-performance-test.sh
[Tests] Minor fixes in run-performance-test.sh

- Use valid value for backend service. Only "HTTP" and "MQTT"
is allowed. If no default is specified, the shell script
will exit with an error
- Remove unused fi

Change-Id: I0ecf172b41bded9eb6a64ab43f6421911833c85f

clive-jevons/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,clive-jevons/joynr