core: Make explicit version of maven war plugin

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

Change-Id: I75a6a4e6bc164a01448b73fae33f36e60a7c9c31
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>
(cherry picked from commit d1b25b148aab648a78ea5c6d9bcc948b27527512)