Rework version artifact version tracking

This patch reworks the way we track our internal artifacts by moving the
definitions from the aprent pom into a dedicated yangtools-artifacts
versions.

This artifact's dependencyManagement section is then imported into the
parent. This may look unnecessary, but it allows our downstream users to
import the same file and have the versioning work.

An interesting side-effect of that is that the properties cannot be
relied on carrying versions of artifacts and then be picked up by the
features.xml file.

This patch fixes that by introducing a script into the build cycle,
which looks at a template features.xml and resolves the versions as
currently visible in the dependency tree of the artifact.

Change-Id: I193586f777478c12520358b9909cfb61e40d14b4
Signed-off-by: Robert Varga <rovarga@cisco.com>
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>