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?
I can do that, of course :) 
I have taken this code from coreos/etcd (https://github.com/coreos/etcd/blob/master/server/tls_info.go#l27)...
I am not sure about ssl certificate, but I think that a tls without the root CA is useless in our case. If your agree on that, I can change the code to enforce this point.