The OpenStack client utilities consume a set of input environment
variables for things like username and auth_url, so it's very
common for OpenStack users to have such settings set in their
environment. Indeed, things like devstack also output a shell file
to be sourced to set them. Although in a playbook it's entirely
expected that variables should be used to pass in system settings
like api passwords, for ad-hoc command line usage, needing to pass
in five parameters which are almost certainly in the environment
already reduces the utility.

Grab the environment variables and inject them as default. Special care
is taken to ensure that in the case where the values are not found, the
behavior of which parameters are required is not altered.