[alerta] token header prefix

Back in December I merged a pull request that changed the alerta Authorization prefix from Bearer to Key because of an issue experienced by @emirozer https://github.com/influxdata/kapacitor/issues/1076.

I was running a custom version of alerta at the time so I assumed that I had changed Bearer to Key myself. On further inspection I realized that alerta officialy started using the Bearer prefix a year ago. https://github.com/guardian/alerta/commit/46e52094013f0c19a664f3aeb7ebdf1bfcbf64f4 which means @emirozer was using an older version of alerta.

I have been running a custom version of kapacitor for myself that uses Bearer so I haven't had any issues, but going forward there is a chance that someone will run into this again. I am proposing that the prefix be configurable. The default value would be Bearer.

I will work on a pull request in the coming days that addresses this. I will be sure to alert @emirozer if the pull request gets accepted.

No file or test is named. Start by locating the Alerta request construction and configuration handling, then verify that the Authorization prefix is configurable with Bearer as the default; done means both prefix variants can be selected without custom code.

Written by the indexing model from the issue text.