Adding retry logic for validate vopt task
Adding retry logic for validate vopt task

The validate_vopt_repo_exists task may fail during parallel scale
operation where multiple threads can lead to accessing the same
entry wrapper and returning HTTP 412 (ETAG MISMATCH). This adds
retries for such scenarios.

Change-Id: I52c433079536c9783319b8a52727dbc4187e9772

pypowervm/pypowervm,powervm/pypowervm