shell: Allow choosing Python version

LISA_PYTHON can be set to any binary name prior to sourcing init_env.
This allows choosing the version of python to use, in case multiple
versions of python are installed on the system, especially for testing.

Cleanup the LISA banner to contain useful information only.

Also change the default name of LISA's venv so it includes the version
name.

NOTE: Since the default name of LISA's venv has changed, it will
reinstall pip packages when sourcing init_env. To avoid this, you can
run the following script before fetching the new version:

ver=$(python3 -c 'import sys; print("{}.{}".format(*sys.version_info))')

old_venv=.lisa-venv3
new_venv=.lisa-venv-$ver

mv "$old_venv" "$new_venv"
ln -s "$new_venv" "$old_venv"