Update 'namespace' priority for 'find_pod' function

[MaksYermak]

In this PR I have updated the namespace priority for the find_pod function. Because we faced the bug, in case when the user specified namespace in the task and inside the metadata for pod_request_obj user has a different namespace. Then namespace will be overwritten by the value which is specified in the task. This priority looks incorrect, because in the find_pod function we need to find a Pod which has already started. And information about this Pod inside the pod_request_obj variable. It means that firstly we need to check value in pod_request_obj.metadata.namespace and, then, value inside self.namespace.

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.