Feature/3.0/xmlimportandvariables

Add the possibility to add variables, so you can say stuff like:

             "<hazelcast>\n" +
                        "    <semaphore name=\"${name}\">\n" +
                        "        <initial-permits>${initial.permits}</initial-permits>\n" +
                        "        <backup-count>${backupcount.part1}${backupcount.part2}</backup-count>\n" +
                        "        <async-backup-count>${notreplaced}</async-backup-count>\n" +
                        "    </semaphore>" +
                        "</hazelcast>";
 
For more info see:
https://github.com/hazelcast/hazelcast/issues/407

The XmlConfigBuilder will by default be initialized with the System properties, but one can inject its own properties as well.

This pr does not contain functionality for imports. That will be part of another pr.