*: use create_venv_dir() helper

*: use create_venv_dir() helper

* define create_venv_dir() helper to
  1. create a temp dir
  2. set a trap to clean it up upon exit
* use create_venv_dir() in all scripts where
  TEMPVENV and/or VENV variables are used.

this change can help us to avoid creating a tempdir in /tmp,
and keep the tempdir forever.

Signed-off-by: Kefu Chai <kchai@redhat.com>