Share default overrides between modules and backends

[mudler]

Currently there is no way to override from backends e.g. defaults that comes from modules - this has the downside of having hacks in modules for the specific backends.

dbb9c9f is an e.g. of how it could be done - read a generated default.sh which is the result of all the concat defaults - allowing in this way to refer to each other freely (or something like that)

Refer to #99