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?
It would help testability if you were passing in io.Readers rather than filenames. At that point, you can just generate objects on the fly and assert the proper *tls.Config is generated.