Don't try to cleanup volume that doesn't exist.
Don't try to cleanup volume that doesn't exist.

Updates the test_volume_create_get_delete test so that it no
longer tries to cleanup when the volume variable is None.

This resolves UnboundLocalError errors which can occur in
the test suite when volumes aren't getting created correctly.

Change-Id: I483219c4bb78c9dbc21be7a974bd208014ec1740

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