Add optional functionality to print real-time output of scripts running during customization step 

Hi,

In our project we use OZ to create heavily customized images - during customization stage we run lot of puppet. It would be really useful like to have live output from OZ customization scripts - to see where we are and whats going on inside virtual machine. 
This is also part of our continuous integration - so we would like to have as much output as possible. Currently oz-customize/oz-install reports stderr as part of exception body only when customization fails, which is useful but not enough.

With this commit I'm trying to add optional parameter "--follow" which will simply print stdout/stderr of all subprocesses running during customization.
I'm not sure if "follow" is a proper name, also not sure if i took right approach to propagate this parameter.
I've successfully tested it on RHEL6.