Archive tempest.conf safely as tempest_conf.txt

* functions.sh(cleanup_host): Instead of renaming tempest.conf to
tempest.conf.txt and then immediately deleting it because it matches
the *.*.txt glob, use tempest_conf.txt instead.

Change-Id: I255a7472ce07b1fedfd15a50fa8d80a24a79f368
Merge "Archive tempest.conf safely as tempest_conf.txt"