fix(kuma-cp): wait between the proxy termination and its deregistration

[lobkovilya]

As a result, there was no sleep between the proxy termination and its deregistration.

Checklist prior to review

• Link to relevant issue as well as docs and UI issues --
• This will not break child repos: it doesn't hardcode values (.e.g "kumahq" as a image registry) and it will work on Windows, system specific functions like syscall.Mkfifo have equivalent implementation on the other OS --
• Tests (Unit test, E2E tests, manual test on universal and k8s) --
• Do you need to update UPGRADE.md? --
• Does it need to be backported according to the backporting policy? --
• Do you need to explicitly set a > Changelog: entry here or add a ci/ label to run fewer/more tests?

[lahabana]

@lobkovilya I think the title of the PR should be what it fixes. .i.e: "wait between the proxy termination and its deregistration"
This is because this is what lands on the changelog users care about behaviour not actual change. (You can expand on the change in the commit content)