JBIDE-13987 properly load proxy wizards from remote discovery source

With central providing offline proxy wizards by default, the discovery mechanism saw a contributor mismatch between all proxy wizard providers.

The fix is fairly simple, though not entirely satisfying, as it exclude processing of proxy wizards contributed by the central plugin during remote discovery.

PR is significant because the ITs had to be fixed in 2 ways :
- all proxy wizard tests were excluded during tycho builds (due to some stupid non default conventions we have in jbosstools : all tests need to be manually declared in a test suite)
- while running tycho builds, remote discovery would fail for some reason (file:/ scheme not being supported by httpclient). Maybe an issue with the TP. Quicker solution was to use http://, served by an embedded jetty server during tests.