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 think you missed my comments on this function. I'll reiterate:
1. Drop the `Get` from the function name
2. Add some unittests of this function