Need more entries in EventSeverity

What's interesting about this is while LogEntry defines Severity to follow the "Health" enum, other properties in the data model (like Severity in Message and in Event) all define it as a string, but the LongDescription points to the Status definition (where Health is defined). What's also interesting is in our published message registry files, we're following the Health enum (OK, Warning, and Critical).

Changing Severity in LogEntry to simply be a string like in other portions will likely cause interoperability issues (since vendors have the chance to define their own terms).

We may want to preserve the Health enum for the sake of keeping Status clean, but add a new enum definition to use for Log (and probably Event, Message, and some other locations) that contains the Health enum plus some more members (like Emergency and Notice). Will need to see if this breaks existing implementations though; the LongDescription language does at least call out "Status", but it's not a hard tie to the Health enum.