Fix java download/install

The configurable tmpdir broke automatic java download and install because it omitted a slash between /tmp and the jdk name.  The result was permission denied:

```
Creating tmp directory (/tmp) if it does not exist
Verifying that java exists
bash: java: command not found
Installing Java
/tmpjava1.6.0_12.tgz: Permission denied
Failed to download Java
```

src/main/java/hudson/plugins/ec2/ssh/EC2UnixLauncher.java

@@ -151,12 +151,12 @@ else if (bootstrapResult == SAMEUSER) {
                 String path = "/hudson-ci/jdk/linux-i586/" + jdk + ".tgz";
 
                 URL url = computer.getCloud().buildPresignedURL(path);
-                if(conn.exec("wget -nv -O " + tmpDir + jdk + ".tgz '" + url + "'", logger) !=0) {
+                if(conn.exec("wget -nv -O " + tmpDir + "/" + jdk + ".tgz '" + url + "'", logger) !=0) {
                     logger.println("Failed to download Java");
                     return;
                 }
 
-                if(conn.exec(buildUpCommand(computer, "tar xz -C /usr -f " + tmpDir + jdk + ".tgz"), logger) !=0) {
+                if(conn.exec(buildUpCommand(computer, "tar xz -C /usr -f " + tmpDir + "/" + jdk + ".tgz"), logger) !=0) {
                     logger.println("Failed to install Java");
                     return;
                 }