Previously, the topologymanager would simply fall back to the None() policy
if an invalid policy was specified. This patch updates this to return an
error when an invalid policy is passed, forcing the kubelet to fail
fast when this occurs.
These semantics should be preferable because an invalid policy likely
indicates operator error in setting the policy flag on the kubelet
correctly (e.g. misspelling 'strict' as 'striict'). In this case it is
better to fail fast so the operator can detect this and correct the
mistake, than to mask the error and essentially disable the
topologymanager unexpectedly.
