nvp_plugin does not support metadata if host doesn't support namespaces

Fix proposed to branch: master
Review: https://review.openstack.org/24562

Reviewed: https://review.openstack.org/24562
Committed: http://github.com/openstack/quantum/commit/891a860d58e651812bb4aa978946159fac3b709a
Submitter: Jenkins
Branch: master

commit 891a860d58e651812bb4aa978946159fac3b709a
Author: Aaron Rosen <email address hidden>
Date: Fri Mar 15 14:45:42 2013 -0700

    Add metadata support for nvp plugin without namespaces

    If using a host that does not support namespaces the nvp plugin did not have
    a way to leverage metadata unless using enable_isolated_metadata which
    requires a subnet to not have a gateway. This patch changes the metadata
    configuration for the NVP plugin by adding a flag metadata which can be
    set to access_network (which was previously enable_metadata_access_network)
    or dhcp_host_route. When set to dhcp_host_route when the dhcp agent creates
    it's port a host_route is added towards the dhcp server's ip for the
    destination 169.254.169.254/32 only if the subnet has as gatway_ip.
    In order for a host to leverage metadata via dhcp_route_route it requires
    the host's dhcp client to support host routes (code 121).

    Fixes bug 1155786

    Change-Id: I2ff52adc3dfebcdd6d9c05d5dc79aa65ae100c74