Startup scripts refer to non-set environment variables YARN_PID_DIR and HADOOP_PID_DIR

[stigviaene]

The startup scripts start-xx.sh rely on YARN_PID_DIR or HADOOP_PID_DIR being set, but this is not guaranteed when the recipes have not run in the same terminal window.
They are set in set-env.sh, but that file is not sourced before running the startup scripts, leading to pid files not being found.