Opsgenie notifier should allow to configure different target address

[erms77]

The opsgenie service uses several regions depending on the location of their client. For European customers the url to use is api.eu.opsgenie.com.

Currently the configuration of a second address is not possible and only the address of the US region is used (api.opsgenie.com)

The opgenie notifier should allow the user to configure the opsgenie address to use.