more cli code deduplication, improved provider registration & easy workdir switching 

Hi Patrick,

I've done some more refactoring. I moved nearly all provider CLI methods (including sendkeys) to the parent class so they are available for all providers. I tested the changes with virtualbox yesterday and they are working fine. And will do some testing with kvm this weekend. Can you test the fusion and the parallels part with may branch ? - If not maybe you know someone who can test. Now the CLI makes full use of the nice provider API and this makes it easier to modify existing cli methods, keep the options more consistent and add new methods for all providers.

Furthermore I added a class option --cwd to set the working directory. This makes it possible to switch the workdir without touching the VEEWEE_DIR environment variable.  
I grepped through the code and saw a few more environment variables. Are they yet documented? If not I can document them - which document/section would you recommend?

Have a nice weekend,
Ruben