[Alerta] Do not set status, fix rawData and default severity is indeterminate

[satterly]

Some minor fixes to the Alerta integration...

• let the Alerta server determine the alert status otherwise alerts that have been acknowledged in the console will be incorrectly modified
• change incorrect data attribute to rawData
• default severity should be indeterminate otherwise automatic status changes don't work as expected
• CHANGELOG.md updated
• Rebased/mergable
• Tests pass
• Sign CLA (if not already signed)

[nathanielc]

@satterly Thanks for the PR, can you help me understand how Alerta determines the status?

[nathanielc]

Now that you have on open PR can you reference the PR here in the CHANGELOG?

[satterly]

@nathanielc From the Alerta docs website...

Alerta is called state-based because it will automatically change the alert status based on the current and previous severity of alerts and subsequent user actions.

The Alerta API will:

• only show the most recent state of any alert
• change the status of an alert to closed if a normal, ok or cleared is received
• change the status of a closed alert to open if the event reoccurs
• change the status of an acknowledged alert to open if the new severity is higher than the current severity
• update the severity and other key attributes of an alert when a more recent alert is received (see correlation)
• update the trendIndication attribute based on previousSeverity and current severity with either moreSevere, lessSevere or noChange
• update the history log following a severity or status change (see alert history)

See http://docs.alerta.io/en/latest/server.html#state-based-browser for more info.

[nathanielc]

@satterly Perfect. Thanks.