This enables a smooth transition to PSP. Today, users would have to
manually set --allow-privileged to true before transitioning to PSP,
which isn't a smooth deprecation path for the flag (we want people
to *stop* setting it). This PR makes the default behavior isomorphic
with what will happen after the flag is removed.

Defaulting --allow-privileged to true should be safe, because it simply
allows a superset of Pods to run (all workloads continue to work).

WRT https://github.com/kubernetes/kubernetes/issues/58010#issuecomment-383264473
the --allow-privileged flag is effectively useless for security, so this
shouldn't be a concern from that perspective.

I also bumped the deprecation timeline in the comment to 1.13.0, so that
we give people the full period of time to stop setting
--allow-privileged, now that the behavior makes it possible to do so.