Add support for the performance plugin.

Throws an error when invalid values are provided.

Also fix a minor spelling error.

Change-Id: Ib32fffbbdab89f072894e675a43eecfb0fff983d
Reviewed-on: https://review.openstack.org/28916
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins