[CDH5] Ensure $USER is set in environment

Some build environments might not have $USER set in the environment.
Various cluster creation routines depend on it.  Forcibly set it to
"id -un" if not set.

Change-Id: Ie0d9abb6a22226c86793623270ad436068e5fbcd
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4208
Reviewed-by: Ishaan Joshi <ishaan@cloudera.com>
Tested-by: Michael Yoder <myoder@cloudera.com>