Feature Request - Allow custom Alias definition for OpsGenie V2 integration

I have a need to use the "alias" field to deduplicate alerts going to OpsGenie. This is especially important for load balanced resources where I may receive 8 separate alerts for a single proxy going down.

I noticed in code that you set the alias ...
`alias := base64.URLEncoding.EncodeToString([]byte(entityID))`

I think I saw in the same code that EntityID might be an option that I could supply within my TICK script but did not see that in the latest documentation. If EntityID is something I could set myself then I suppose encoding the byte stream would still be OK. I also make use of the Chronograf UI to build my tasks but see nowhere to add EntityID via the UI; however, if that's a valid option then I could just update the TICK script, I suppose.

Start by locating the OpsGenie V2 integration code that assigns alias from the encoded entityID. Review how EntityID is exposed in TICK scripts and whether Chronograf can configure it; done means defining and documenting a supported way to provide a custom deduplication alias.

Written by the indexing model from the issue text.