Upgrade to Commons Configuration v2

Just after we updated our Configuration System to use Apache Commons Configuration v1 (in DS-2654), version 2 was released.

https://commons.apache.org/proper/commons-configuration/userguide/upgradeto2_0.html

While the features are not significantly different, version 2 has better integration with Spring (which was not out-of-the-box in v1): https://commons.apache.org/proper/commons-configuration/userguide/howto_utilities.html#Use_Configuration_in_Spring

The upgrade itself is not difficult, as most of the code is contained in dspace-services (with a little in dspace-api). It also has a lot of unit tests which can verify the upgrade "works" as expected.

I've got a PR in the works.