Clean-up version properties in kie-parent-metadata

 * use kie.version property as default for others like drools.version
   and jbpm.version.

 * the kie.version is also used for kie-bom to be able to change
   version in submodule without interfering with the version of
   kie-bom (before, the version could not in fact be changed,
   unless the kie-bom was also released with that updated version)
Should I change the version here (and below) to ${kie.version} too? I am not sure as for example the other jbpm* versions probably should contain ${jbpm.version}.