MAR-173: Switch to using Prosody 0.11 as default version in the xmpp_server role:

- Updated default value for the Prosody package parameters.
- Configure the backports repository on the server, and pin the
lua-ldap package to be installed from the backports
repository (needed for Lua 5.2 support).
- Drop the explicit installation of lua-sec library - it is already
installed as pre-requisite for the Prosody package.