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?
Ok. I'll keep this logic.
To come back to my code, I don't make dangerous assumptions, it's completely wrong :)
Switching from filepath to file content (either Io.Reader or []byte) helps testability but remove the logic of using keys or not from the test.
I will correct my code and test with generating temp files.