Define default value for ActiveMQ client thread pool

Artemis in EAP 7 is creating client thread pool with current default size of 500. This value appears to be too high and leads to consumption of system resources. OS has limitations for number of threads/processes.

This value should be adjusted to common needs.

There is related feature request to allow configuration of this value in EAP 7 - JBEAP-3748

Currently this value can be modified only by setting system property provided at start of EAP server:
-Dactivemq.artemis.client.global.thread.pool.max.size=<value>

Thread is initialized in Artemis on line: https://github.com/rh-messaging/jboss-activemq-artemis/blob/1.1.0.wildfly-013/artemis-core-client/src/main/java/org/apache/activemq/artemis/api/core/client/ActiveMQClient.java#L221