Support OpsGenie Priority field

[Tom-Fawcett]

Now that Alertmanager is on the OpsGenie v2 API we can use the "Priority" field.

This is a non-mandatory field described as follows:

Priority level of the alert. Possible values are P1, P2, P3, P4 and P5. Default value is P3.

Further information about this field can be found here, including information on validation:

If an unknown value is assigned or the tool does not carry a priority information, the alert will have the default priority value, P3.

With the above in mind, this PR does not validate the value for this field. Though I could add validation if so desired.

[stuartnelson3]

Seems fine to me. If there's confusion we can add validation later.

[Tom-Fawcett]

Thanks :)