Provide an option to configure Weld global thread pool size

As mentioned in https://issues.jboss.org/browse/WFLY-4653 it could be useful to have an option to configure Weld thread pool size e.g. in standalone configuration. Currently the size of the thread pool is hardcoded at https://github.com/wildfly/wildfly/blob/master/weld/src/main/java/org/jboss/as/weld/services/bootstrap/WeldExecutorServices.java#L51
However note that this thread pool is used ONLY during the bootstrap of the Weld container.