Drop the local bin/pip check to use pip found in the PATH
Use `pip install tables` instead of the git version
Drop manual tables package removal since moving away from git
Remove TABLES_GIT variable
Replace pip virtualenv installation by message and script exit

Since python_deps.sh is now standalone, we do not know where where are in the
filesystem and do want to install a virtualenv anywhere on the disk. If no pip
is found in the PATH,  a warning message is displayed asking to install pip
and the script is terminated.
Also use pip directly rather than cd'ing to the parent directory and using
bin/pip.
Merge pull request #906 from sbesson/rebased/develop/python_deps_simplifications

Python deps simplifications (rebased onto develop)