Avoid swallowing exceptions in network manager

In _allocate_fixed_ips() and setup_networks_on_host(), we spawn off
green threads to call _rpc_allocate_fixed_ip() and
rpc_setup_network_on_host() on remote network managers, but if there is
an exception in the thread it just gets swallowed.

We don't really need a GreenPool here. It's not like its limit of 1000
threads is relevant here. Just use eventlet.spawn(), collect the list of
threads spawned and call wait() on each of them. This will ensure the
exception gets raised by wait().

Also, fix a case in the v3 integrated servers test where we are setting
up fake networks which are associated with a non-existent 'fake-mini'
host. See also I9854863bbec8834c638c55832411f63da570f82d.

blueprint: oslo-messaging
Change-Id: I7af1c4c1869edf3601809fe02fb82b12c8b70811
Merge "Avoid swallowing exceptions in network manager"