etcdctl/v3: add lease keep-alive --once flag

[marcovc]

Fixes: #8719

[xiang90]

lgtm. @gyuho do we need an e2e test for it? if yes, can you point @marcovc to the place to add the test?

/cc @jpbetz

[gyuho]

Yeah, simple e2e test case would be good. We can write one around https://github.com/coreos/etcd/blob/master/e2e/ctl_v3_lease_test.go#L75.

[marcovc]

Sorry, I didn't got it. Am I supposed to write the test? If yes, then should I create a second pull request?

[gyuho]

@marcovc You can add another commit for the test. Thanks!

[marcovc]

@gyuho, I added the test, although as far as I understand it simply tests if the command runs, and not if it does what is expected. But so does the other tests in that file. I called the "test" binary but I'm not sure the test was called. Let me know if you need anything else.

[gyuho]

lgtm. defer to @xiang90