Declaring core modules should be done through external properties

Currently, the core modules are declared in the ModuleConstants.java through a Map in which the module id and version needs to be edited. This will be easier to maintain if core modules can be declared in external properties file, as different developers/implementers would want to package OpenMRS differently and bundle other core modules without which OpenMRS should not start.

Thus, for easy configuration of distribution, we should support declaring core modules, through properties file.