Volume test for volume_extension:volume_actions:upload_public

This commit adds a test for testing the policy action:
volume_extension:volume_actions:upload_public.

Specifically:
  - adds test for volume_extension:volume_actions:upload_public
    whose minimum microversion is 3.10 [0]
  - changes the image_client for related test for test_volume_upload
    to use the admin image client for waiting for status change
  - tags this test and related test_volume_upload test as slow
    since they both take 20 secs or more to complete

[0] https://github.com/openstack/cinder/blob/a98a16a658244814c079b01a0c4d05e2a1deca43/cinder/api/contrib/volume_actions.py#L272

Change-Id: I2d69b48202b4dc6b4ef4adcb5bf2388353ccbd9d
2 files changed