BUG 2302 : odl-clustering-test-app should not be part of the odl-restconf-all feature set

This patch does the following,

1. It removes odl-clustering-test-app from the restconf features
2. Move odl-clustering-test-app to the md-sal feature (just like odl-toaster)
3. Removes all unneccessary dependencies from odl-clustering-test-app
4. Removes all the clustering configuration files from the odl-clustering-test-app feature

This now allows the odl-clustering-test-app to be used with/without clustering. It also
eliminates the problem is inadvertently installing the odl-clustering-test-app when installing
odl-restconf-all.

Change-Id: I8021f88c967576aa8cf8da54dbfa8f4464233bc3
Signed-off-by: Moiz Raja <moraja@cisco.com>