add metadata-agent network role

...and the accompanied entries in role[os-network] and
infrastructure.yml.

It seems like this was forgotten in the earlier commit,
323843237c10e5a8ebf245bc23ff159f9fe311a5, at least the OVS plugin of
Neutron needs this agent.

Change-Id: I69c0543597624497c9a40e731e355b8f64d5b3fb