Support nets with capsules

Per [1], Zun does not accept nets in capsules even though it has some code for that [2].
Per docs [3], all containers share the same network but, since it cannot be set, it gets to be purely random (the first from network listing). This limits the usefulness of capsules.

[1] https://review.opendev.org/747964
[2] https://opendev.org/openstack/zun/src/commit/1f7419b2ba49ecdf964f2ec524ec839543d412ea/zun/api/controllers/v1/capsules.py#L221
[3] https://opendev.org/openstack/zun/src/commit/1f7419b2ba49ecdf964f2ec524ec839543d412ea/doc/source/contributor/capsule.rst#user-content-capsule-quick-start