In the SshOnlyProvisioner get the user name from the provisioner's property instead of a system property

The `SshOnlyProvisioner` gets the user name from the system variable `fallout.system.user.name` and for some reason that is empty. However, it should fetch the username from the `user.name` property of the sshonly provisioner. This fix addresses datastax#9

Author: kijanowski

src/main/java/com/datastax/fallout/components/common/provisioner/SshOnlyProvisioner.java

@@ -126,7 +126,7 @@ protected CheckResourcesResult reserveImpl(NodeGroup nodeGroup)
     protected boolean startImpl(NodeGroup nodeGroup)
     {
         return nodeGroup.waitForAllNodes(node -> {
-            String userName = FalloutPropertySpecs.userPropertySpec.value(node);
+            String userName = userPropertySpec.value(node);
             String host = sshHostSpec.value(node);
             Integer port = sshPortSpec.value(node);
             String password = userPasswordSpec.value(node);