core: Add external event error messages need to be improved

Add external event error messages need to be improved

Adding EventValidator class to validate severity enum values.
The flood_rate can be omitted and the default is set to 30
(this was working prior to this patch)

Change-Id: I743ff20bd3d4317bc054ba314ada6bd918a7502f
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=949551
Signed-off-by: Eli Mesika <emesika@redhat.com>