Increase number of php-fpm workers

With this change we overwrite the apache2-base php fpm pool config
with our own custom file, which we'll then have to maintain.

conf/php/7.0/fpm/pool.d/www.conf

+[www]
+listen = /run/php/php7.0-fpm.sock
+
+pm = dynamic
+pm.max_children = 40
+pm.start_servers = 10
+pm.min_spare_servers = 10
+pm.max_spare_servers = 20
+pm.max_requests = 1000
+
+pm.status_path = /status
+
+; Redirect worker stdout and stderr into main error log. If not set, stdout and
+; stderr will be redirected to /dev/null according to FastCGI specs.
+; Note: on highloaded environement, this can cause some delay in the page
+; process time (several ms).
+; Default Value: no
+catch_workers_output = yes
+
+access.log = /proc/self/fd/2
+
+chdir = /
+
+; Already enabled by default - do not enable twice.
+; php_admin_value[opcache.enable] = 1
+php_admin_value[opcache.memory_consumption] = 64
+php_admin_value[opcache.interned_strings_buffer] = 16
+php_admin_value[opcache.max_accelerated_files] = 4000
+php_admin_value[opcache.validate_timestamps] = 0
+php_admin_value[opcache.fast_shutdown] = 1
+