[5.x] Implement default environment

[driesvints]

This PR implements a default environment for Horizon. Any other environment will re-use this default environment's settings which can be overridden. Supported environments still need to be defined but this helps with the maintenance burden of having to maintain duplicate settings for multiple environments.

I've updated the config file which immediately shows how this can be useful.

Sent to master because it's obviously breaking for anyone who was relying on the current formatting of the config.

Closes #438

[driesvints]

Additional options for this PR are:

• Move the defaults out of the environments array into a separate config option (might be more clearer)
• Don't require environments to be defined in the array and use the default one when one cannot be found

[hivokas]

Thanks @driesvints! That's a very useful feature.

[setkyar]

Hi @driesvints , I added the documentation for this PR. Since it's confusing for people who use multiple supervisors and the default queues are on there. Please help check and help me add comments if I can improve on the documentation.

Thanks