health: make Opsgenie API URL configurable (#10561)
* Update health_alarm_notify.conf Add OPSGENIE_API_URL="" to specify API endpoint * Update health_alarm_notify.conf Add URL for sanity * Update alarm-notify.sh.in Add default value of https://api.opsgenie.com for OPSGENIE_API_URL when unset Substitute OPSGENIE_API_URL for URL on line 2182 * Update alarm-notify.sh.in Re-added parenthesis deleted in error on ln 2180 * Update README.md Amend to reflect OPSGENIE_API_URL variable * Update health_alarm_notify.conf Remove hardcoded URL as now has default if unset * Update alarm-notify.sh.in Moved OPSGENIE_API_URL=${OPSGENIE_API_URL:-"https://api.opsgenie.com"} from line 394 to line 415 as per comment https://github.com/netdata/netdata/pull/10561#discussion_r568430266