Ignored socket-level keepalive flag in OpenJDK ORB connections

Restored the ability to use socket-level keepalive flag in OpenJDK ORB connections after it became impossible in Wildfly 11.

Wildfly version 11 brought in the following change: 09b79c4#diff-cabdeec060c4801f721b35660a6081a3R347 which starts to implicitly declare the property com.sun.CORBA.transport.ORBSocketFactoryClass to value org.wildfly.iiop.openjdk.security.NoSSLSocketFactory due to default value of the iiop-openjdk wildfly setting support-ssl being false (see iiop-openjdk/src/main/java/org/wildfly/iiop/openjdk/IIOPSubsystemAdd.java).
But the code which handles the keepalive property is only found in DefaultSocketFactoryImpl of opendk-orb module, so it is basically never executed.