Pass agent work_dir to isolator modules

Some isolator modules can benefit from access to the agent's work_dir. For example, the DVD isolator (https://github.com/emccode/mesos-module-dvdi) is currently forced to mount external volumes in a hard-coded directory. Making the work_dir accessible to the isolator via Isolator::recover() would allow the isolator to mount volumes within the agent's work_dir. This can be accomplished by simply adding an overloaded signature for Isolator::recover() which includes the work_dir as a parameter.