Enable Network Namespaces by Default for GRE

Previously, the instructions in the OVS GRE install guide
had network namespaces explicitly disabled, based on historical
documentation.  However, this is no longer valid.  Instructions
on disabling network namespaces have instead been moved to a general
note, and the instructions for enabling network namespaces have
been bumped up from the OVS VLAN-specific instructions to the
general OVS instructions.

Cherry-Picked from: https://review.openstack.org/#/c/53380/
Change-Id: Icd2ea7ffcf6f412d66efd1f46a386318ae74e135
Fixes: bug #1241056
backport: havana