To fix test_get_dss_rp in test_vmware_vmdk.py

The test "test_get_dss_rp" in test_vmware_vmdk.py is not testing
the positive case as intended. Rather, it tests the negative case
(no datastores) which is already covered by
"test_get_dss_rp_without_datastores".

Change-Id: I5b4802e22218217e0338fc721fc0ce1efeaee4f1
Closes-Bug: #1251126
Remove dead code from test_get_volume_stats()

The _emulate_vgs_execute() routine is never called because the function
that calls it (get_all_volume_groups) is overridden to return fake
output without executing vgs.

Change-Id: I3d3f536f7d85128547dc9e758e3930412ebed121
Add greenthread.sleep() to parent wait()

Add an eventlet.greenthread.sleep() to wait() to keep
cinder-volume parent from looping after catching SIGTERM
and hogging the CPU. This was already added to the running
state but is missing from the SIGTERM caught state when
the parent waits to reap the child.

Change-Id: Ia871e31db5bf9ec4e86f926f8f6c4f0f4ecb7925
Closes-Bug: #1254089
Update openstack/common/periodic_task

The following commits are in this update:

1771a77 Adjust import order according to PEP8 imports rule
4bfb7a2 Apply six for metaclass
0ea069e Removed calls to locals()

Change-Id: Ied20ce735af79b52e7674a3dfe3b097b84b61533
Merge "Update openstack/common/periodic_task"
Merge "Add greenthread.sleep() to parent wait()"
Merge "Remove dead code from test_get_volume_stats()"
Merge "To fix test_get_dss_rp in test_vmware_vmdk.py"