This attempts to fix#3676. `PrevNoExist` checks if the key previously exists
and if so, it returns an error, which is how `mk` command is supposed to work.
The previous code ignores the previous key and overwrites with the later value.
/cc @yichengq
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.
