mediawiki: move SSHD nice override from web.pp to init.pp

It's a bit bizarre for SSHD to be nice'd -10 on the web appservers but not the
rest. Applying it across all app servers makes things more uniform.

::ssh::server is already included on all servers because it's part of ::base.

Change-Id: I37694e4ab186d2ac9ca60c4b1fcd75e38ad9bbd8