OpenStack: Reuse an existing OpenStack client, don't make an auth req in every mcm method call

[kayrus]

What would you like to be added:

Currently an authed OpenStack client is created for each OpenStack API call, this is not efficient. You can create an authed OpenStack client at startup and reuse it. For example you can take a look on how it is done in k8s openstack controller: https://github.com/kubernetes/cloud-provider-openstack/blob/8afa0f628e15a3016b2add62d53f96d4921a6a0a/pkg/cloudprovider/providers/openstack/openstack.go#L488..L554

Why is this needed:

This will reduce a load on keystone API and significantly lower an amount of API calls.

/cc @hardikdr @rfranzke @vpnachev

[kon-angelo]

This issue was moved here from the old MCM repository. Hence I would like to re-examine it's validity for the current MCM and out-of-tree providers.

Here are my concerns:

• the credentials can change at any time, including during the runtime and we would need to deal with credential updates. Therefore, simply creating at startup is not enough.
• The credentials are part of the machineclass and theoretically each machineclass can use different ones. Therefore we would need to support multiple authed clients.

It is a bit different usecase from CCM that gets restarted when credentials get updated. Since creation on startup is not an option for us, I think the next best thing would be to create and cache the clients per hashedCredentials or a similar scheme.