mediawiki: make apache MaxClients be always smaller than ServerLimit

In a preceding change, we decided to dynamically calculate MaxClients
for out servers based on the potential memory consumption. This resulted
in a very large number on larger-ram servers. However, MaxClients cannot
be larger than ServerLimit, which is 256 at the moment.

ServerLimit has therefore been parametrized and used in calculating
MaxClients as an upper limit. We may decide to raise it in the future
only on larger RAM systems.

Change-Id: Ic55b35c51fd2956af1068c6606142b78d6df6838
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>