Add metadata support for nvp plugin without namespaces

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