Trigger node deletion when machine is deleted

[gagan16k]

What this PR does / why we need it:

1. Machine controller now explicitly deletes the backing node from the target cluster on machine delete events, and removes finalizers from nodes that are being deleted with no backing machine.
2. The machine status description during creation has been updated from "Creating machine on cloud provider" to "Creating machine on cloud provider. Waiting for node object to register", so the status accurately reflects which stage the machine is in.

Which issue(s) this PR fixes:
Fixes #1044, #1064

Release note:

MCM now deletes the backing node when its machine is force-deleted, and clears finalizers on nodes terminating without a backing machine. The machine creation status is also modified to distinguish cloud-provider failures from node-registration failures.

[gardener-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign ashwani2k for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment