Export plugin warnings to DB so that they could be listed later in web UI

[qdelance]

Plugin execution may lead to warnings (this is the case today with Centreon Plugins), these warnings are displayed in stderr. This output is not redirected anywhere at the moment so a user cannot see them.

The goal is to redirect these messages in logs table en DB, users will then be able to see them close to other Engine logs.

To be discussed : find a way (new flag?) in DB to make it possible to filter them from other logs? If so, Web is likely to be impacted, so we will need to create a new ticket/issue in their tracker.