CleanUp poms

Remove all <groupId>org.apache.struts</groupId> from children's poms as defined in parent pom groupId is enough.

Also define all children in parent pom to allow reuse them in other child pom without specifying version number:

<dependency>
    <groupId>org.apache.struts</groupId>
    <artifactId>struts2-convention-plugin</artifactId>
    <version>${project.version}</version>
    <scope>test</scope>
</dependency>