Add option to configure Grafana Host and API key per room

[kstevena]

Following #90 I'm proposing to add:

• a new environment variable to specify the user want to enable per room/channel configuration of the grafana endpoint
• a new command to set the configuration of the grafana host and api_ki and use robot brain to store it.

The purpose is to be able to use a single instance of the bot within different channel, and each would have a different Grafana instance used for Grafana related queries.

It does not do / support the following:

• check if the user updating the conf is part of an "Admin" list of users allowed to do that
• support multiple Grafana backend for a single room / channel

[kstevena]

Thanks @stephenyeargin, I did not notice sendError prior to your suggestions ;)

[stephenyeargin]

@kstevena Released as part of v2.3.0 :tada: