Edit server/default/conf/gatein/configuration.properties and comment out next rows in JCR section:

#gatein.jcr.datasource.driver=org.hsqldb.jdbcDriver
#gatein.jcr.datasource.url=jdbc:hsqldb:file:${gatein.db.data.dir}/data/jdbcjcr_${name}
#gatein.jcr.datasource.username=sa
#gatein.jcr.datasource.password=

and in IDM section:

#gatein.idm.datasource.driver=org.hsqldb.jdbcDriver
#gatein.idm.datasource.url=jdbc:hsqldb:file:${gatein.db.data.dir}/data/jdbcidm_${name}
#gatein.idm.datasource.username=sa
#gatein.idm.datasource.password=

In jcr-configuration.xml and idm-configuration.xml comment out the plugin InitialContextInitializer.

<!-- Commented because, Datasources are declared and bound by AS, not in eXo -->

<!--

<external-component-plugins>

    [...]

</external-component-plugins>

-->

Running eXo after these configurations goes well.