Keep a singleton to support multiple get_config calls
Keep a singleton to support multiple get_config calls

We are destructive to os.environ in the OpenStackConfig constructor- so
it really should only ever be called once. Make sure get_config does
this.

Change-Id: I279bdf68408a807ec18fba634df3769c9b8fc4dc
Closes-Bug: #1691294

openstack/python-openstacksdk,openstack/os-client-config,dtroyer/python-openstacksdk,stackforge/python-openstacksdk,openstack/python-openstacksdk,dtroyer/python-openstacksdk,stackforge/python-openstacksdk,dtroyer/os-client-config