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)
Unreachable due to check 2 lines above. Both `cert` and `key` are set, or none of them are set.