Fix security vulnerabilities using Bandit

Tempest was using:
-python module xml.etree[1],

It was vulnerable to different atacks.

Instead of xml.etree.ElementTree tempest is now using
defusedxml.ElementTree which is more secure.

[1] https://bandit.readthedocs.io/en/1.7.0/blacklists/blacklist_calls.html B313

Change-Id: I50a8ab3c3be2decccd7480ecf00f1a3e4a75f172
diff --git a/requirements.txt b/requirements.txt
index c4c7fcc..a118856 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -21,3 +21,4 @@
 PrettyTable>=0.7.1 # BSD
 urllib3>=1.21.1 # MIT
 debtcollector>=1.2.0 # Apache-2.0
+defusedxml>=0.7.1 # PSFL