This commit adds support of setting config options to the exec plugin
from cli.
Next options are added:
* --exec-command new command for the exec credential plugin
* --exec-api-version API version of the exec credential plugin.
* --exec-arg new arguments for the exec credential plugin command
* --exec-env add, update or remove environment values for the exec credential plugin
Moved all flag code from `staging/src/k8s.io/apiserver/pkg/util/[flag|globalflag]` to `component-base/cli/[flag|globalflag]` except for the term function because of unwanted dependencies.
