✨ v1beta2: Add managedNetworks on OpenStackCluster

[nikParasyr]

/kind feature

As part of the v1beta2 api improvements:

Group network related fields under managedNetwork

For most of the resources that capo creates we have a field with managedXYZ. This is not the case for network, so:

Kind: OpenStackCluster
...
spec:
  ...
  disablePortSecurity: false/true
  networkMTU: <int>
  ...

Should be grouped into:

Kind: OpenStackCluster
...
spec:
  ...
  managedNetwork:
    disablePortSecurity: false/true
    mtu: <int>
  ...

Implementation should be done after core v1beta2 is implemented in #2895

[nikParasyr]

/assign