core: Make EJB dependency in root explicit

The EJB dependency is needed by the  WelcomeServlet class.
Explicitly defining it in pom.xml makes the pom easier to understand
and solves some hickups some IDEs have with it.

Change-Id: I621ec30aea05ddce6f9cce97b637198ecb76b77a
Signed-off-by: Allon Mureinik <amureini@redhat.com>