[CI] Ensure network is set for Zun

If we don't set it, then Zun chooses one randomly (the first one
from Neutron).
This may break if it is a network that is not available on
target hosts, e.g. external via L3 agent router.

Since capsules do not support nets yet [1], this patch ensures
desired network creation order in init-runonce instead.

[1] https://bugs.launchpad.net/zun/+bug/1895263

Change-Id: Iaa113dcfb826164a2772d2c91d34ec0236be0817