core: Consolidate engine.version

Currently there are two properties that contain the version of the
engine, and the value is assigned in multiple places. This patch changes
the POM files so that this property is defined only once, in the root
POM.

Change-Id: I1b58568d9106eea0841653e9882f4b19b8aea4ee
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>