add support for passing args in env vars.

For all plugins where it makes sense. Added for glance, keystone,
novaapi the usual openstack variables - OS_AUTH_URL, OS_USERNAME,
OS_PASSWORD, OS_TENANT_NAME, OS_REGION_NAME.

Added utils.py module containing a common Action class which wraps the
envvar collection in argparse.

Fixes #1.
Merge pull request #2 from rochaporto/new/envvar-support#1

add support for passing args in env vars.