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?
If the user is clearly attempting to configure TLS, a cert and a key must always be provided. Some people care about a custom CA, but some won't, so I'd prefer not to make that a requirement.