Simplify using external routers and metadata

The dhcp agent only pushes out the metadata
static route when the subnet is isolated, and it
determines that by checking if the gateway_ip is
not set. This makes it tricky to use external
routers and metadata from dhcp at the same time.

This patch changes how the dhcp agent determines
that the subnet is isolated. It now considers it
isolated if there is no Neutron router on it.
This makes it straightforward to use an external
router on a provider network and get the
metadata from the dhcp namespace.

Change-Id: I0e29a2f058564c267176dab26da00f6ef579808b
Closes-Bug: 1236783
(cherry picked from c73b54e50b62c489f04432bdbc5bee678b18226e)
Merge "Simplify using external routers and metadata" into stable/havana