Fix DeprecationWarning of jsonschema

`jsonschema.draft4_format_checker` and also `jsonschema.FormatChecker.cls_checks`
are deprecated and DeprecationWarning appears every time so these make
logs a little dirty and also these attribute/classmethod will be removed
in the future.

Therefore `jsonschema.draft4_format_checker` should be replaced with
`jsonschema.Draft4Validator.FORMAT_CHECKER` and
`jsonschema.FormatChecker.cls_checks` should be replaced with
`jsonschema.Draft4Validator.FORMAT_CHECKER.checks`.

Eventually, DeprecationWarning for jsonschema will disappear with this
proposed fix.

Closes-Bug: #2008490
Change-Id: I1ef326b8ac17f4bbcbc718d13223da156b0e5ec3
diff --git a/tempest/lib/common/jsonschema_validator.py b/tempest/lib/common/jsonschema_validator.py
index 1618175..5212221 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.draft4_format_checker
+FORMAT_CHECKER = jsonschema.Draft4Validator.FORMAT_CHECKER
 
 
 # NOTE(gmann): Add customized format checker for 'date-time' format because:
@@ -39,7 +39,7 @@
         return True
 
 
-@jsonschema.FormatChecker.cls_checks('base64')
+@FORMAT_CHECKER.checks('base64')
 def _validate_base64_format(instance):
     try:
         if isinstance(instance, str):