webadmin,userportal: Specify GWT module in pom.xml

It might happen that gwt-maven-plugin auto-discovers
other (support) modules (like GwtCommon) besides the
main application module (like WebAdmin). This patch
specifies the main module explicitly in pom.xml file.

Change-Id: I18482f5c69072183501116e55ff36ee603dc8fc0
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>