Fixes https://github.com/apache/pulsar/issues/4956

Master Issue: https://github.com/apache/pulsar/issues/4956

### Motivation

In the latest version of pyyaml, using load will issue a warning and explain some reasons here https://github.com/yaml/pyyaml/wiki/PyYAML-yaml.load(input)-Deprecation.

### Modifications

* Use yaml.FullLoader as default loader

(cherry picked from commit d49e61e0)