Current from-scratch script is bogus

Quite a few issues from the latest merge, mister @amohammadi:

1. The clean-up of the previous environment missed the --yes flag, which I added
2. If the previous environment was tweaked, then the automated script does not work - maybe an option to brute force remove (as with rm -rf) the environment directory would be a plus in case it survives the clean-up. I also added this feature
3. When installing for python 3.x, caffe has a conflict, as it requires python 2.7. The "brute force" method indicated on the commit log does not work automatically. I commented it out and let you solve this one @amohammadi. I still "manually" uncommented that bit for the python2.7 installations we have. So, limited testing is possible.
4. The version of opencv installed alongside python2.7 is 2.4.x, while python3.x is 3.1.x. Is this by design?