core: Make explicit version of maven ear plugin

Currently we don't explicitly specify the version of the maven ear
plugin that we use. This patch makes it explicit in the plugin
management section of the root POM.

Change-Id: Ia0718385df3f3cc8e4113210294890c774de6209
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>
(cherry picked from commit e7efd1f61f1ac879fb0a0a62a91539592189d6d0)