Eliminate warnings in powervm driver
Eliminate warnings in powervm driver

To get nova_powervm.virt.powervm.driver warning-free:

o Refreshed method signatures from ComputeDriver.
o Added explicit import of taskflow.task.
o Made a couple of private methods static.

No functional changes.

Change-Id: I96a7c2c34b2ea6f37beeded601fc11824c1a3ce0

openstack/nova-powervm,openstack/nova-powervm,stackforge/nova-powervm,stackforge/nova-powervm