Improvements on example script and README

• make instructions consistent with web page

https://scitasadm.epfl.ch/confluence/display/DOC/How+to+use+Tensorflow+on+the+GPU+nodes

• do not use --user for pip install as we are already in a virtualenv

• drop hardcoded paths on example script (it works from the git repo)

• add checks for existence of virtualenv in script