proxy simple requests

Before, `start`, `kill`, `pause` and `unpause` used `dockerclient`, there is no real advantages and it added lots of boilerplate code.

Instead, use a generic proxy method. That better because:
* it's faster, for example for kill we don't have to extract the `signal` parameter anymore
* when docker gets updated, we don't have to wait for `dockerclient` to be updated.
* as it's generic I was able to add `stop`, `restart`, `exec/json`, `resize`, `wait` and `exec/resize` with one line

Also, I removed all the 302 redirect to use this "proxy". This means only the master is required to connect to the nodes.
Your cluster can be on a different network that the docker cli, as long as you have access to the master it'll work.
Is this log line required? If so, could it be a Debugf instead?