Remove duplicate env function in shell.py
Remove duplicate env function in shell.py

There already exists an env() function in utils. Let's use that
one since it's common.

Change-Id: I661984394cf0c0543b2f35bf76e3929dead54d1d

varunarya10/python-openstackclient,openstack/python-openstackclient,BjoernT/python-openstackclient,redhat-openstack/python-openstackclient,BjoernT/python-openstackclient,varunarya10/python-openstackclient,dtroyer/python-openstackclient,openstack/python-openstackclient,dtroyer/python-openstackclient,redhat-openstack/python-openstackclient