Enable SSL Certificate authentication for etcd endpoint

This pull request allows to connect etcd with certificate authentication. 
Change are made for fleetctl and fleet

Some code could be factored between etcd and fleet... but I need this functionality now. 
Could you merge before refactoring?
You're making some dangerous assumptions in here. Calling a function should not have wildly different behavior based on the arguments. It should do one thing. Split this function in two:

`createTLSClientConfig(cert, key []byte)` and `setTLSClientConfigRootCAs(*x509.CertPool)`