Remove POWERVM_ATTRS from vm.py
Remove POWERVM_ATTRS from vm.py

For maintainability, use ATTRS_MAP.keys() instead of the redundant
POWERVM_ATTRS.

Change-Id: I16fe0d910e9046c05f2658fc54888c77abaaae1c

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