Revert "Fix DeprecationWarning of jsonschema"

This reverts commit d52fb8cb6d2ed8893bc36f2f1f786cd890b1732e.

Reason for revert:

We need to support till xena as Tempest master do support stable/xena onwards branches. stable/xena has jsonschema 3.2.0 only https://github.com/openstack/requirements/blob/stable/xena/upper-constraints.txt#L548

We need to avoid such bumping unless something is failing.

With jsonschema 3.2.0 we do not have the new things so let's keep them and we can fix it once we have jsonschema version as upper-constraint where they are going to be removed.

https://github.com/python-jsonschema/jsonschema/blob/v3.2.0/jsonschema/validators.py#L475

Change-Id: Iab8dbcd469e2ca234c9a42673b8b02f470ec2404
diff --git a/tempest/lib/common/jsonschema_validator.py b/tempest/lib/common/jsonschema_validator.py
index 5212221..1618175 100644
--- a/tempest/lib/common/jsonschema_validator.py
+++ b/tempest/lib/common/jsonschema_validator.py
@@ -18,7 +18,7 @@
 
 # JSON Schema validator and format checker used for JSON Schema validation
 JSONSCHEMA_VALIDATOR = jsonschema.Draft4Validator
-FORMAT_CHECKER = jsonschema.Draft4Validator.FORMAT_CHECKER
+FORMAT_CHECKER = jsonschema.draft4_format_checker
 
 
 # NOTE(gmann): Add customized format checker for 'date-time' format because:
@@ -39,7 +39,7 @@
         return True
 
 
-@FORMAT_CHECKER.checks('base64')
+@jsonschema.FormatChecker.cls_checks('base64')
 def _validate_base64_format(instance):
     try:
         if isinstance(instance, str):