Add a test bean to try dedicate id generator per entity class

We found since EclipseLink 2.5.0, the following feature does not
support.

http://stackoverflow.com/questions/18943310/why-does-eclipselink-2-5-0-d
oes-not-support-override-tablegenerator-or-sequence

We just live with it, maybe eclipselink will release the feature in the
future. Otherwise just push down the Id field to concrete classes with
specific @GeneratedValue and @TableGenerator defined