Move disk.py module to util module

Current location for disk.py modules is inconsistent with the
usage of utils for basic operations on disks, not only their
metadata. Non-metadata operations must be implemented by util
module.

Change-Id: Ia53b831f4b4d5c31da8801a8561b4721fd35d511
Related-bug: 1585938
Related-bug: 1575054

octane/handlers/install/compute.py

@@ -14,8 +14,8 @@ import logging
 import os.path
 
 from octane.handlers import install
-from octane.helpers import disk
 from octane import magic_consts
+from octane.util import disk
 from octane.util import node as node_util
 from octane.util import plugin
 from octane.util import ssh

octane/handlers/upgrade/compute.py

@@ -16,8 +16,8 @@ import stat
 import subprocess
 
 from octane.handlers import upgrade
-from octane.helpers import disk
 from octane import magic_consts
+from octane.util import disk
 from octane.util import env as env_util
 from octane.util import node as node_util
 from octane.util import plugin