The classic local filesystem source is still supported
using the same traditional configuration key, load_definitions.
Configuration schema follows peer discovery in spirit:
* definitions.import_backend configures the mechanism to use,
which can be a module provided by a plugin
* definitions.* keys can be defined by plugins and contain any
keys a specific mechanism needs
For example, the classic local filesystem source can now be
configured like this:
``` ini
definitions.import_backend = local_filesystem
definitions.local.path = /path/to/definitions.d/definition.json
```
``` ini
definitions.import_backend = https
definitions.https.url = https://hostname/path/to/definitions.json
```
HTTPS may require additional configuration keys related to TLS/x.509
peer verification. Such extra keys will be added as the need for them
becomes evident.
References #3249