role::mediawiki::webserver: set maxclients dynamically

The api and bits application servers already set maxclients to 100. I don't
think there's a principled reason for the very particular values we assign to
maxclients on the standard app servers -- they seem like carryovers from old
configs. This is especiall silly now that the bits app servers are just generic
app servers (following change I51c6a6c33).

So instead derive an optimum MaxClients value by setting aside 30% of RAM for
the OS, APC, and misc. services, and divide the remaining 70% by the average
memory cost of an Apache worker. Take the minimum of the result and 100.

Change-Id: Ia424c6433fb87b565a4bb81cf3eeadce7bac3655