Allow more customizations in config.js

### Background and Motivation

Grafana can be configure in many different ways, but the basic setup is always the same.

In order to use your module,
I want to pass in my own config-template
But still use the class to define the essential bits from the manifest.

### Implementation

I added `*_scheme`-parameters for `graphite`, `elasticsearch`, and `influxdb`. I have Graphite and ElasticSearch running on a https-host, so I need that ability.

I also do not use InfluxDB, so I made the config-template overridable. This allows me to use our own template. In the template, I can still use the parameters defined in the class.

### Side effects

Having the template overridable is essentially giving people enough rope to hang themselves. Then again, the target audience is admins and devops-teams, so we're all capable of handling this.