Unix commands interprets argument value of first character '-'
as a flag. And this won't be fixed in our upstream CLI libraries.
This adds help message to show users workarounds.
Addressing https://github.com/coreos/etcd/issues/2020.
Like the commit 8ebc9331111, this commit lets simple etcdctl commands
use a context with timeout value passed via -total-timeout.
This commit doesn't change complex commands like watch,
cluster-health, and import because it is not obvious that using the
context in the commands is good or not.
