codegen: Use Maven to generate code

Currently the code generator has to be executed as a Java class with
a script or from an IDE. This patch adds a Maven profile that executes
the generator using the exec plugin, so that it can be executed with the
following command:

  mvn validate -Pupdate-code

Once the generated code is updated the developer is resposible from
adding it to the source code repository.

Change-Id: I2b3472ed3009aa6acc2500d49e26628660764aaf
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>