admin.novarc missing OS_ on a couple of variables

[Alvaro Uria]

On templates/admin.novarc,
"""
export PROJECT_ID={{ credentials_project_id }}
export PROJECT_DOMAIN_ID={{ credentials_project_domain_id }}
"""

...should be,
"""
export OS_PROJECT_ID={{ credentials_project_id }}
export OS_PROJECT_DOMAIN_ID={{ credentials_project_domain_id }}
"""

OTOH, API_VERSION is not recognized by OpenStack. Only OS_<component>_API_VERSION, so the following should be removed:
"""
export API_VERSION={{ credentials_api_version }}
"""

[Alvaro Uria (aluria)]

Pushed to the charmstore cs:prometheus-openstack-exporter-1