use existing networks, fallback to network/subnet defined in cluster …

[sbueringer]

This PR enables the following:

• reuse existing networks/subnet if they cannot be created (by defining NodeCIDR in the OpenStackCluster)
• define network/subnet on the cluster level so it doesn't has to be defined in every Machine (only works for one net/subnet not multiple)

Fixes #428

[k8s-ci-robot]

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[sbueringer]

/assign @jichenjc
/assign @hidekazuna
/hold

I didn't test it yet myself but I'm on it :)

[sbueringer]

Okay I tested my existing multi-node setup and I tested it with remove networks filter. Both worked

/hold cancel

@jichenjc Can you test if you're now able to use loadbalancers by setting the network and subnet fields (filters) in the ClusterSpec?

[sbueringer]

Was fixed in #477