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

requirements.txt

@@ -21,3 +21,4 @@ stevedore>=1.20.0 # Apache-2.0
 PrettyTable>=0.7.1 # BSD
 urllib3>=1.21.1 # MIT
 debtcollector>=1.2.0 # Apache-2.0
+defusedxml>=0.7.1 # PSFL

tempest/lib/services/object_storage/account_client.py

@@ -14,8 +14,8 @@
 #    under the License.
 
 from urllib import parse as urllib
-from xml.etree import ElementTree as etree
 
+from defusedxml import ElementTree as etree
 from oslo_serialization import jsonutils as json
 
 from tempest.lib.common import rest_client

tempest/lib/services/object_storage/container_client.py

@@ -14,9 +14,9 @@
 #    under the License.
 
 from urllib import parse as urllib
-from xml.etree import ElementTree as etree
 
 import debtcollector.moves
+from defusedxml import ElementTree as etree
 from oslo_serialization import jsonutils as json
 
 from tempest.lib.common import rest_client