[GAL-336] Adding system-paths setting to feature-pack

[spyrkob]

Issue: https://issues.redhat.com/browse/GAL-336

related galleon-plugins PR: https://github.com/wildfly/galleon-plugins/pull/234

[spyrkob]

@jfdenise, I wasn't sure if the version of feature pack XSD needs to go up with this changes. If it does, let me know and I'll update the PR

[jfdenise]

@spyrkob , yes we should introduce a 3.0 xml and schema.

[spyrkob]

@jfdenise updated with galleon-feature-pack 3.0

[spyrkob]

@aloubyansky / @jmesnil could you review this please?

[aloubyansky]

@spyrkob please update the docs to point people to the description of this feature.

[spyrkob]

@aloubyansky I expended the Handling user-managed content section to describe how the system-paths work and added example to Feature-pack metadata (feature-pack.xml). Are there other sections that should be updated?