Added REST API support for getting list of loaded and all modules. Tinkered with Web UI's flow action display. The problem is that the actions (write and apply) will not display for any flow. Need to spend some time figuring out the issue.

Clean up serialization for REST APIs.  Get rid of the jackson converter module and just use annotations everywhere when appropriate.  There may still be a desire for something like the jackson converter module but it didn't really mesh with the plugin system, and the annotations work nicely.

Initial support for OVS-style role request messages and maintaining connections with switches in a backup role [#23366119].
Added REST API to get/set the current role of the controller [#2336329].
Added simple health check REST API [#23364299].
Fixed off-by-one error in the message code values for the role request/reply messages.

Use Java properties (either via config file or command line) for per-module specific configuration files. Note: This obsoletes previous command line configuration properties apart from -cf (configuration file).

Fixed JUnit tests.