Automatic merge from submit-queue (batch tested with PRs 38990, 45781, 46225, 44899, 43663)

Fix command exec -- COMMAND can not contain spaces

Fixes https://github.com/kubernetes/kubernetes/issues/7688
the problem is when you execute command:
 `cluster/kubectl.sh exec -p client-blue-8yw37 -c client -i -t -- 'ls -t /usr'`
the args is 
[`client-blue-8yw37` , `ls -t /usr`] 
**instead of** 
[`client-blue-8yw37`, `ls`, `-t`, `/usr`]
@kubernetes/sig-cli-pr-reviews, so I add a warning, wdyt ?
cc @ymqytw @adohe @fabianofranz