Integration test: provide config file as command line argument

[onnozweers]

Currently the integration test tests/integration_test.sh uses a fixed configuration file tests/test.conf.

As dCache admins, we'd like to use Ada's integration test for automatic monitoring. We then would like to be able to use multiple config files; one for each API interface we want to test. So, we'd like to be able to run tests/integration_test.sh prod.conf or tests/integration_test.sh test.conf.

test.conf could be the default in case no file is specified; then it would be backwards compatible.