Use absolute remoteFS for OpenShiftSlave.

According the javadoc this should be an absolute path. Using relative one works in some situations but results in errors using tools installers for instance:

```
Unpacking http://archive.apache.org/dist/maven/binaries/apache-maven-3.0.5-bin.zip to app-root/data/jenkins/tools/hudson.tasks.Maven_MavenInstallation/maven-3.0.5 on rawbldrbldr
FATAL: command execution failed
java.io.IOException: Cannot run program "app-root/data/jenkins/tools/hudson.tasks.Maven_MavenInstallation/maven-3.0.5/bin/mvn"
```

In a perfect world this should be passed to superclass' constructor but the uuid does not seem to be available yet.