update machineclasses to use cloudprovider secret

[kon-angelo]

How to categorize this PR?

/area control-plane
/kind enhancement
/priority normal
/platform openstack

What this PR does / why we need it:

Uses the machine classes .{spec.}credentialsSecretRef field. It is a reference to a secret containing only the credentials, while today's .spec.secretRef still contains the user-data. See here for more details.

Uses the worker's secret reference as the credentialsSecretRef.
This means all worker pools use the same "cloudprovider" secret containing only the cloud provider credentials.
The existing MachineClass SecretReference then only contains the user data that is different for each pool.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
Set to draft, due to the pending discussion in gardener/gardener-extension-provider-aws#238.

Release note:

Reducing credential update complexity by all the machine classes using the new .{spec.}credentialsSecretRef field.
This means all worker pools use the same "cloudprovider" secret containing only the cloud provider credentials.
The existing MachineClass SecretReference only contains the user data that is different for each pool.

[gardener-robot]

@kon-angelo Labels area/todo, kind/todo do not exist.

[prashanth26]

/assign

[prashanth26]

/lgtm
@kon-angelo - The PR looks good from my side. You can open it for review from other colleagues.

needs/rebase

[rfranzke]

Love it :)

/lgtm

[dkistner]

/lgtm

[prashanth26]

/lgtm

Thanks.