Openstack provider clean-up

[irozzo-1A]

This issue is to do some clean-up in the openstack provider logic, that in some cases is not very clear or optimal. A few examples:

In order to get a network by name or ID we iterate through all networks instead of getting it directly with the proper API call:
https://github.com/kubermatic/machine-controller/blob/master/pkg/cloudprovider/provider/openstack/helper.go#L217

Same applies for security groups:
https://github.com/kubermatic/machine-controller/blob/master/pkg/cloudprovider/provider/openstack/helper.go#L184

The logic we use to get the default subset is a bit obscure: if we have just one subnet in the network provided in input we return it otherwise we obtain the subnets from the API and we return the first if any or we throw an error. If there is a reason behind that, I have some doubts, we should at least comment it properly.
https://github.com/kubermatic/machine-controller/blob/master/pkg/cloudprovider/provider/openstack/helper.go#L416

[kubermatic-bot]

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/lifecycle stale

[kubermatic-bot]

Stale issues rot after 30d of inactivity.
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/lifecycle rotten

[xmudrii]

/remove-lifecycle rotten
/kind cleanup