OpsGenie v2 Support

[nathanielc]

Comparing https://docs.opsgenie.com/docs/alert-api to https://docs.opsgenie.com/docs/alert-api-deprecated it looks like there are a few difference:

• The authentication method changes from a property of the JSON body to an HTTP header
• The team/recipients properties are combined into a respondents list. Teams are {"name":"team name","type":"team"} and recipients are {"username": "user name","type":"user"}.

At this point I believe we can switch to using the new API without needing to create a new config or handler. Simply updating the existing code should be enough.

[russorat]

link #1708