remove disk type validation

[kon-angelo]

What this PR does / why we need it:
Removes strict disk type validation. The main reason is to support gardener/gardener-extension-provider-gcp#702. As providers add more disk types the validations need to be adapted.

The PR could go either way, of adding the check for the new type or removing the current check and I was in favor of the later approach.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

Remove strict validation about disk types

[rishabh-11]

If we remove the validation and the disk type does not exist, it will be known only during the VM creation and not before that. Is that fine for the customer?

[kon-angelo]

From gardener perspective there is an additional validation done from cloudprofile. So the user will discover it about the same time. But by removing this validation, next time I need to add a disk type, I only need to do it on cloudprofile level.

For a standalone MCM use, the question is if these disks are different than say vm types, which we accept as input, but do not validate against.

[rishabh-11]

/lgtm

I think in the standalone case, it should be fine if we don't have the validation, the machine creation call will error out and that should be sufficient