There was an issue with the metadef APIs in glance, detailed in
the latest OSSN[0] that they have the potential to leak resources.
This change updates the default policy for the metadef APIs to
be disabled by default.
[0] https://wiki.openstack.org/wiki/OSSN/OSSN-0088
Change-Id: I7377b3a2f3784fe7da78bdd7aba146328cc0f406