Current etcdctlv3 doesn't handle options related to security when they
are not passed. Connections cannot be established and etcd produces
logs like this:
```
14:09:07 etcd1 | 2016/02/04 14:09:07 transport: http2Server.HandleStreams received bogus greeting from client: "\x16\x03\x01\x00\x9a\x01\x00\x00\x96\x03\x03\xf6\t\xda\x06QV\xb4\xdd\xc1gF\x1cC"
```
This commit fixes the problem. In addition, a case that empty strings
are passed to the options (e.g. --key="") are treated as error.
