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?
What I was trying to say before, is that if keyfile is provided but certfile is not, we cannot proceed. It is unclear whether or not the user intends for TLS encryption be used.