webapp: improve access log parsing
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)

SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site