avocado: Remove references to /var/tmp/avocado

This probably won't work for multi-users and/or
if /var/tmp/avocado exists before avocado is run
for the first time (with different permissions from
a different user). It's also probably a security
issue: a malicious user could create a /var/tmp/avocado
symlink pointing to /home/someone/important-dir.

This patch removes references to /var/tmp/avocado,
fixing this important security bug.

Recommended reading:

https://kurt.seifried.org/2012/03/14/creating-temporary-files-securely/
http://www.linuxsecurity.com/content/view/115462/151/

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>