Avoid using v3 volume clients when v3 is disabled

In the BaseVolumeTest class, some v3 specific volume clients are
accessed even if this API version is not enabled.

This change ensures we avoid using them if v3 API is disabled,
preventing an AttributeError from being raised.

Change-Id: I0d1f4bfda468d05287dec02137c1842184e66e30
Closes-Bug: #1714228
1 file changed