Add notification timeout

[benoit74]

Rationale

All requests operations should have a timeout otherwise the code is waiting indefinitely for an HTTP response.

Notifications (at the end of a task) must have such a timeout.

Default timeout is small (5 secs) to avoid issues if not set (no reason to wait for a process which is "fire-and-forget" by design).

An HTTP timeout during a notification is catched and logged as an error but will not stop further notification to proceed and will not stop the changes to be commited to the DB.

This is a tentative to enhance situation on #817.

Changes

• add a constant / environment variable named `` with default value 5
• use this for slack, webhook and mail notifications