Add request durations in access logs and improve logstash's integer parsing

Several commit to limit the review :

webapp: code format

webapp: improve access log parsing

• fix a hardcoded path to the apache log file, filebeat was not reading it when the vhost name is not webapp.
• add an environment field to easily identify the origin of a log entry
• add a vhost field to easily regroup the logs of a vhost (can be also added on varnish side)

logstash: Add support an optional duration on apache logs

It will allow to monitor the front responses time per requests. The conversion of the request and duration fields to integer is mandatory to use them in some aggregation. The ES indexes will have to be reindexed if we want to have the full history.

apache: Add the request duration on access logs

It will add some visibility on the monitoring

Related to T2787

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Migrated from D4989 (view on Phabricator)