Don't spawn metadata-proxy for non-isolated nets

Don't spawn metadata-proxy for non-isolated nets

If the configuation option "enable_isolated_metadata = True" for the
DHCP agent is set, the neutron-ns-metadata-proxy process is spawned
for all networks, regardless if they are isolated or not. In case
the network is not isolated (ie. connected to a neutron router), the
L3 agent also spawns a proxy process, and the DHCP's proxy is left
unused. This patch adds a check prior to the spawning of new proxies:
if a network is not isolated, no proxy is spawned.

Change-Id: I9bdb8c3d37997b22435bca33ec47a67db08efa51
Closes-bug: #1361545