TLS version and ciphers

[fredericmoulins]

Address the base part of #253.

• Configure Mozilla's Intermediate TLS profile for postfix, dovecot, nginx and ejabberd servers.

• Use standard Diffie-Hellman parameters from RFC7919. Remove the possibility to configure DH parameters size less than 2048.

• Adapt the isync IMAP client configuration for TLS.

• Factor the TLS parameters (ciphers, TLS versions, OCSP stapling, server preferred order) in a security.tls object in the default configuration.

The parameter for the DH parameters size could be moved to the security.tls object, but it might be nice to have a task linked to the load_defaults role to warn (fail) when someone is not using the new way of configuring it. Kept for later.

The Mozilla's Intermediate profile recommends server_preferred_order: false. It is what is configured here. This is open for discussion.

Not tested on buster yet.

More details in each commit description.

[arodier]

Thanks for this, it seems to add better security. I was not aware of the RFC, I am still reading the documentation.