Change the way JUnitSnmpAgent creates agents

* Allow getting the port from the MockSnmpAgent
* Use port 0 when creating mock agents in the executor
  so that it will pick a free ephemeral port
Merge remote-tracking branch 'origin/1.12'

Conflicts:
	core/test-api/snmp/src/main/java/org/opennms/core/test/snmp/JUnitSnmpAgentExecutionListener.java
	tests/mock-snmp-agent/src/main/java/org/opennms/mock/snmp/MockSnmpAgent.java