Rename trust extension.

Change trust extension from RH-TRUST to OS-TRUST so that the namespace
being used is for OpenStack, as opposed to a contributing company.  This
is also more consistent with namespacing used in other OpenStack APIs.

Some additional discussion about this is in this thread:
http://lists.openstack.org/pipermail/openstack-dev/2013-March/006876.html

Change-Id: I0fd869abe0f527c899808a4dde19dbd1fb6f32cd