Need to empty /tmp/*-env.sh, before appending

Defensively empty the *-env.sh files in case they are ever
appended to multiple times.

append_oozie_setup function simply append to /tmp/oozie-env.sh.

We'd better initial oozie-env.sh with empty content first.

The same apply for generate_setup_script function.

Change-Id: If9c02e85856272d0b48ef7d79e2f329c58aa148b
Close-bug: #1235481