Make tmp directory location on slave configurable

I created new functionality to be able to configure the tmp directory location on the slave, which is used for running the init script.
Due to the security policy of my company to not allow execute permissions on the /tmp folder I was unable to start an instance. That's why I started modifying the plugin. I think this might be useful for others as well (for example for fixing JENKINS-19956 Error launching slave using jenkins-ec2 plugin) so please consider adding my changes.

Changes:
A new field is added to the SlaveTemplate configuration, which allows overwriting the tmp directory location. In case no value is provided a default value will be used, which differs between UNIX and Windows
slaves.

![screenshot](https://cloud.githubusercontent.com/assets/9528619/4901322/830221c0-6430-11e4-8537-1b0ef58cd2d8.png)