wmflib: add ssl_ciphersuite

We have our ssl configs in 100 places all over puppet, and converting
everything to PFS was a real pain and involved 100 commits (or more).

Changing anything at the moment would be a mess and require 100 changes
as well, so we're providing a rudimentary, simple solution to output
commonly configured apache and nginx configs for SSL within a centralized
function that will compute the string to add to your manifest/template.

The output of this functions can also be used to generate a separate include
file that will be read by the web server.

Change-Id: I9bc1104b7f770d9f0157b9f81791a534193a4915
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>