tools/setup-virtualenv.sh: do not default to python2.7

tools/setup-virtualenv.sh: do not default to python2.7

This commit changes the script's behavior when run without a --python option.
Before, we were running virtualenv with "--python python2.7" in that case.

That is not a reasonable default in September 2019, given that Python 2 EOL is
coming on January 1, 2020. Also, it breaks setup-virtualenv.sh in py3-only
environments.

With this patch, the script will not pass any "--python" option to virtualenv
(i.e., it will let virtualenv do whatever it thinks is best) in this case.

Fixes: https://tracker.ceph.com/issues/41821
Signed-off-by: Nathan Cutler <ncutler@suse.com>