Ensure cluster identity in deletion flow

[rfranzke]

How to categorize this PR?

/area quality
/kind bug
/priority critical

What this PR does / why we need it:
The cluster identity is required to ensure the kube-apiserver service, but it wasn't computed in shoot deletion flow. Hence, if a shoot already has a deletion timestamp and Gardener v1.8 gets deployed then the Gardenlet will panic because the cluster identity is missing:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x18ac35d]
goroutine 59999 [running]:
github.com/gardener/gardener/pkg/operation/botanist.(*Botanist).setAPIServerAddress(0xc00a0d0bd0, 0xc00c868af0, 0x4a, 0x2084b80, 0xc00e67b110)
	/go/src/github.com/gardener/gardener/pkg/operation/botanist/controlplane.go:1416 +0x80d
github.com/gardener/gardener/pkg/operation/botanist.(*Botanist).DefaultKubeAPIServerService.func1(0xc00c868af0, 0x4a)
	/go/src/github.com/gardener/gardener/pkg/operation/botanist/controlplane.go:1363 +0x77
github.com/gardener/gardener/pkg/operation/botanist/controlplane.(*kubeAPIService).Wait.func1(0x206eec0, 0xc0270f53e0, 0xc01bcce950, 0xc00a567ab0, 0x13fc637)
	/go/src/github.com/gardener/gardener/pkg/operation/botanist/controlplane/kube_apiserver_service.go:143 +0x1c1
github.com/gardener/gardener/pkg/utils/retry.UntilFor(0x206eec0, 0xc0270f53e0, 0xc020e72e40, 0x2059000, 0xc01bcce950, 0xc01d4d4b70, 0x1be8680, 0xc0270f5301)
	/go/src/github.com/gardener/gardener/pkg/utils/retry/retry.go:146 +0x3e
github.com/gardener/gardener/pkg/utils/retry.(*ops).Until(0xc0002e5ce0, 0x206eec0, 0xc0270f53e0, 0x12a05f200, 0xc01d4d4b70, 0xc01bcce940, 0x1)
	/go/src/github.com/gardener/gardener/pkg/utils/retry/retry.go:190 +0xa2
github.com/gardener/gardener/pkg/operation/botanist/controlplane.(*kubeAPIService).Wait(0xc00e2a03f0, 0x206eec0, 0xc0270f53e0, 0x0, 0x0)
	/go/src/github.com/gardener/gardener/pkg/operation/botanist/controlplane/kube_apiserver_service.go:136 +0x18a
github.com/gardener/gardener/pkg/operation/botanist/component.(*deploy).Deploy(0xc01f9748a0, 0x206ee80, 0xc000058018, 0x0, 0x0)
	/go/src/github.com/gardener/gardener/pkg/operation/botanist/component/deploy.go:72 +0xf4
github.com/gardener/gardener/pkg/operation/botanist.(*Botanist).WakeUpControlPlane(0xc00a0d0bd0, 0x206ee80, 0xc000058018, 0x2ad340ce, 0xc00f71e710)
	/go/src/github.com/gardener/gardener/pkg/operation/botanist/controlplane.go:335 +0x32c
github.com/gardener/gardener/pkg/utils/flow.(*execution).runNode.func1(0xc01620ede0, 0x1dadfaa, 0x3d, 0x206ee80, 0xc000058018)
	/go/src/github.com/gardener/gardener/pkg/utils/flow/flow.go:218 +0x1d4
created by github.com/gardener/gardener/pkg/utils/flow.(*execution).runNode
	/go/src/github.com/gardener/gardener/pkg/utils/flow/flow.go:213 +0x14d
runtime: note: your Linux kernel may be buggy
runtime: note: see https://golang.org/wiki/LinuxKernelSignalVectorBug
runtime: note: mlock workaround for kernel bug failed with errno 12

/cc @timuthy
/assign @timuthy

Release note:

A bug has been fixed which could cause a nil pointer exception in case the v1.8 Gardenlet tries to delete a shoot that wasn't reconciled yet.

[vpnachev]

/lgtm

[timuthy]

/lgtm
Thank you!!!