Bug 1099447 - [as7] It's not possible to add a port to SocketBindingGroup resource

'fixed-port' property is not required by EAP, mark it as such in the plugin descriptor

(cherry picked from commit e6c85505ede4077d511b73e55daf0a7cc46163e8)
Signed-off-by: Thomas Segismont <tsegismo@redhat.com>