cipher specification cleanup
https://trac.gajim.org/ticket/7599
Add ability to configure TLS protocol version.

add configuration option tls_version

Related to
https://python-nbxmpp.gajim.org/ticket/17
Backed out changeset 4ba5c2f6386e
I have submitted the 'tls_version' to the incorrect default branch.
The correct submit was done in changeset 15331:b5831b64e5d2 ,
branch gajim_0.16 .
Backed out changeset c1f88dc9e4d0 of backed out changeset 4ba5c2f6386e