[config] Allow using kafka registered at chroot

This PR adds support for kafka registered under a different path from root, as mentioned in the [configuration docs](http://kafka.apache.org/08/configuration.html).
https://github.com/pinterest/secor/blob/master/src/main/config/secor.common.properties should be the right place.

.common.properties is the shared root of all configs. The thinking behind the hierarchy was that people will use the .prod.properties branch for production configs and .dev.properties for running end-to-end tests (e.g., scripts/run_tests.sh)