powervm: revert driver to pass for plug_vifs

Change Ieaf256a5c0f54804686318d6cbd5877a5003c9eb made the powervm driver
plug_vifs method raise NotImplementedError rather than pass, which is
what the hyper-v and vmware drivers do for plug_vifs. This results in
the compute manager failing to init the host because it's not catching
NotImplementedError from driver.plug_vifs.

This patch simply changes the powervm driver plug_vifs method back to
the old behavior (pass), but at least logs a debug message this time.

Closes-Bug: #1217586

Change-Id: Iafe87cbb9b3cf728a47cb9cc43151798fcbb780f
Merge "powervm: revert driver to pass for plug_vifs"