Move Dothill tests out of root test directory

The Dothill code merged but unfortunately had it's
unit tests in the base tests directory.

This patch just moves it to tests/unit/volume/drivers

Change-Id: I010ed2f784384e272599ec3139957989f8838e6f
Closes-Bug: #1462106

diff --git a/cinder/tests/test_dothill.py b/cinder/tests/unit/volume/drivers/test_dothill.py
similarity index 100%
rename from cinder/tests/test_dothill.py
rename to cinder/tests/unit/volume/drivers/test_dothill.py