XD-1811 Custom module registry location

- In addition to `xd.module.home` and `classpath:/modules` as the default
module registry locations, an additional `xd.customModule.home` is provided
to let the user define any specific supported URL path for the module registry.
- Add an entry for `xd.customModule.home` in servers.yml so that user can
specify it.

Fix description not to use http:// location

Handle customModuleHome placeholder

 - Add `isActive()` to ModuleRegistry
 - Include only the active module registries to the DelegatingModuleRegistry
Comment out properties
Assign null default for custom module registry

 - Having `null` as the default value for custom resource module
registry will let us handle the look up methods return null/empty
values.