Integration test suite fails when JAVA_HOME is not set

The build tries to run the kill-server Ant target before and after running the integration test suite. The target assumes that the server is running, and if none of ps/jps/lsof/netstat find the server it fails the build:

Caused by: org.apache.tools.ant.BuildException: The following error occurred while executing this line:
/home/jenkins/workspace/polarion-report/5fee2853/server/integration/src/main/ant/infinispan-server.xml:102: Not yet supported on UNIX/WINDOWS favour without working ps/lsof/jps/netstat

Normally this doesn't happen because the jps check is not as strict as the others, passing if jps finds any java process (and not necessarily a server). But if JAVA_HOME is not defined, the jps check also fails:

[exec] Execute failed: java.io.IOException: Cannot run program "${env.JAVA_HOME}/bin/jps" (in directory "/home/jenkins/workspace/polarion-report/5fee2853/integrationtests/wildfly-modules"): error=2, No such file or directory

We should change the kill-server target so that it doesn't assume the server is already running. Maybe it could also use ps/jps to find the pid of the server, and lsof/netstat to check that the server really is really stopped after it was killed.