Add TLS version & cipher list configuration

These commits add the ability to select a TLS version to use (instead of the hard-coded 1.0), and configure a specific cipher list.

Note: as mentioned in the docstring (changed through one of the commits in this PR), the Python client can't handle any TLS version other than 1.0 due to Python 2.x restrictions.

TODO:
* [x] Extend documentation
* [x] Pass CI (running, `arakoon-git-launcher` 337)
What do you mean with 'impossible'?
That this code path is unreachable or is it a message for the user who did some misconfiguration?
(In the second case I think we might want a better message pointing the user in the correct direction...)