powervm: pass on unimplemented aggregate operations

The powervm driver does not implement the add/remove/undo aggregate
operations defined in the base ComputeDriver class which can cause
issues when running Tempest with the powervm driver. This patch simply
overrides the methods in the powervm driver and does a pass like in the
libvirt driver.

Fixes bug 1206528

Change-Id: Ieac227f321339a4069b015303b6e713ab9fad7b7
Merge "powervm: pass on unimplemented aggregate operations"