bootstrap_system.sh should be able to auto-detect IMPALA_HOME. The only reasonable default for IMPALA_HOME is the repository that the script is being run from. We should detect that, instead of defaulting to ~/Impala, which results in a weird error if you checked out Impala somewhere else.
We should be able to put the minicluster data directory in a different place from the source checkout, e.g. if you want to put it on a different disk.