Remove Node.mCpu, replaced with the ipol vcpu_ratio

Remove Node.mCpu, replaced with the ipol vcpu_ratio

Since the iPolicy contains the vcpu_ratio now, we can remove the
per-node mCpu value (which was the initial way to set this attribute).

Furthermore, we need to re-set the hiCpu value when changing the
policy.

And of course some associated unittest changes.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>