Allow default index mapping customisation

[justincorrigible]

As part of #267, we're making sure Maestro/ES doesn't assign "random" mappings for indices not found.
Those mappings should be made available for customisation per projects' needs, which will be set on "runtime" not just at "build" time.

Note: Considering Maestro generally runs within a container, the injection of these values could be handled through Helm configmaps and/or Docker volumes.