If a data source is configured to use RoleBasedCredentialMapIdentityLoginModule, then exception is thrown at startup [1], because default username and password are null. Please, add module options "username" and "password" to set up default user (similar functionality have e.g. CallerIdentityLoginModule and PassthroughIdentityLoginModule), so DV is able to properly load data source at startup when no user is authenticated and therefore no mapping could be performed.
Example configuration [2]. Note, there is no exception if UsersRoles login module is used instead of RealDirect. However, it means that EAP users are separate from DV users.

FYI:

• credentialMap module option should be defined as URL (file://...). It would be nice to have this information in the documentation.
• I tried to use unauthenticatedIdentity module option for RealmDirect, but same exception has been thrown with different root cause (realm 'ApplicationRealm' not found). I do not know why.

[1]
ERROR [org.jboss.as.connector.subsystems.datasources.AbstractDataSourceService$AS7DataSourceDeployer] (MSC service thread 1-5) Exception during createSubject()PBOX000016: Access denied: authentication failed: java.lang.SecurityException: PBOX000016: Access denied: authentication failed
at org.jboss.security.plugins.JBossSecuritySubjectFactory.createSubject(JBossSecuritySubjectFactory.java:84)
at org.jboss.jca.deployers.common.AbstractDsDeployer$1.run(AbstractDsDeployer.java:1084)
at org.jboss.jca.deployers.common.AbstractDsDeployer$1.run(AbstractDsDeployer.java:1079)
at java.security.AccessController.doPrivileged(Native Method) [rt.jar:1.8.0_40]
at org.jboss.jca.deployers.common.AbstractDsDeployer.createSubject(AbstractDsDeployer.java:1078)
at org.jboss.jca.deployers.common.AbstractDsDeployer.deployDataSource(AbstractDsDeployer.java:600)
at org.jboss.jca.deployers.common.AbstractDsDeployer.createObjectsAndInjectValue(AbstractDsDeployer.java:282)
at org.jboss.as.connector.subsystems.datasources.AbstractDataSourceService$AS7DataSourceDeployer.deploy(AbstractDataSourceService.java:316)
at org.jboss.as.connector.subsystems.datasources.AbstractDataSourceService.start(AbstractDataSourceService.java:120)
at org.jboss.msc.service.ServiceControllerImpl$StartTask.startService(ServiceControllerImpl.java:1980)
at org.jboss.msc.service.ServiceControllerImpl$StartTask.run(ServiceControllerImpl.java:1913)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [rt.jar:1.8.0_40]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [rt.jar:1.8.0_40]
at java.lang.Thread.run(Thread.java:745) [rt.jar:1.8.0_40]

[2]

<security-domain name="my-sec">
    <authentication>
        <login-module code="RealmDirect" flag="required">
            <module-option name="password-stacking" value="tryFirstPass"/>
            <!--<module-option name="unauthenticatedIdentity" value="guest"/>-->
        </login-module>
        <login-module code="org.teiid.jboss.RoleBasedCredentialMapIdentityLoginModule" module="org.jboss.teiid" flag="required">
            <module-option name="password-stacking" value="useFirstPass"/>
            <module-option name="credentialMap" value="file://${jboss.server.config.dir}/teiid-credentialmap.properties"/>
        </login-module>
    </authentication>
</security-domain>