In order to use the PostgreSQL (or any custom) database, you'll need to skip the --container JBOSS_AS7 parameter; otherwise, this will override the data-source configuration as seen in the INFO messages:

***INFO*** Setting transaction-type="JTA"
***INFO*** Using example data source [java:jboss/datasources/ExampleDS]

We should probably make this more clear, or perhaps create a warning or error scenario instead of INFO. When you remove the --container parameter.

It should be much clearer that you may need to use some of the extended parameters like --jndiName and so forth.

This can probably be done by updating the PersistencePlugin and related classes:
https://github.com/forge/core/blob/master/javaee-impl/src/main/java/org/jboss/forge/spec/javaee/jpa/PersistencePlugin.java