Fixed Stop() for external etcd

[makhov]

Description

Don't try to stop etcd in case the external one is used.
Currently, it panics:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xa8 pc=0x23053fc]

goroutine 1 [running]:
github.com/k0sproject/k0s/pkg/supervisor.(*Supervisor).Stop(0x4001257280)
        /go/src/github.com/k0sproject/k0s/pkg/supervisor/supervisor.go:237 +0x20c
github.com/k0sproject/k0s/pkg/component/controller.(*Etcd).Stop(0x400014ac80?)
        /go/src/github.com/k0sproject/k0s/pkg/component/controller/etcd.go:233 +0x24
github.com/k0sproject/k0s/pkg/component/manager.(*Manager).Stop(0x40013e3d40)
        /go/src/github.com/k0sproject/k0s/pkg/component/manager/manager.go:118 +0xe4
github.com/k0sproject/k0s/cmd/controller.(*command).start.func1()
        /go/src/github.com/k0sproject/k0s/cmd/controller/controller.go:302 +0x24
github.com/k0sproject/k0s/cmd/controller.(*command).start(0x400160f770, {0x3b2fd80, 0x40013e3d00})
        /go/src/github.com/k0sproject/k0s/cmd/controller/controller.go:502 +0x3774
github.com/k0sproject/k0s/cmd/controller.NewControllerCmd.func2(0x4000ed2600, {0x400042b2c0, 0x0, 0x2?})
        /go/src/github.com/k0sproject/k0s/cmd/controller/controller.go:114 +0x3a0
github.com/spf13/cobra.(*Command).execute(0x4000ed2600, {0x400042b2a0, 0x2, 0x2})
        /run/k0s-build/go/mod/github.com/spf13/[email protected]/command.go:940 +0x5c4
github.com/spf13/cobra.(*Command).ExecuteC(0x4000c37500)
        /run/k0s-build/go/mod/github.com/spf13/[email protected]/command.go:1068 +0x340
github.com/spf13/cobra.(*Command).Execute(...)
        /run/k0s-build/go/mod/github.com/spf13/[email protected]/command.go:992
github.com/k0sproject/k0s/cmd.Execute()
        /go/src/github.com/k0sproject/k0s/cmd/root.go:194 +0x20
main.main()
        /go/src/github.com/k0sproject/k0s/main.go:43 +0x238

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist:

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[k0s-bot]

Successfully created backport PR for release-1.27:

• [Backport release-1.27] Fixed Stop() for external etcd  #4328

[k0s-bot]

Successfully created backport PR for release-1.28:

• [Backport release-1.28] Fixed Stop() for external etcd  #4329

[k0s-bot]

Successfully created backport PR for release-1.29:

• [Backport release-1.29] Fixed Stop() for external etcd  #4330