Stop using deprecated contextlib.nested
Stop using deprecated contextlib.nested

'contextlib.nested' was deprecated in py2.7 and removed in py3.
So, remove its usage for improvement of py 2/3 compatibility.

Change-Id: I49c71e696ffdeb61ebee308249667739e1b1b0db
Partially-Implements: bp py3-compatibility

vponomaryov/manila,NetApp/manila,sajuptpm/manila,NetApp/manila,openstack/manila,vponomaryov/manila,bswartz/manila,scality/manila,scality/manila,redhat-openstack/manila,redhat-openstack/manila,sajuptpm/manila,jcsp/manila,bswartz/manila,openstack/manila,jcsp/manila