normalize openstack config parameter names.

This normalizes all of the class options for the openstack provider by removing the prepended  "openstack_" string to all config parameters.

When developing tooling for a multi-cloud environment (e.g. openstack and aws), the goal should be to abstract away differences between cloud providers. This is not possible when each fog provider has chosen to add provider-specific nomenclature to their configuration parameters. 

When I specify "region", it should be implied that it means "region" in whatever provider I've instantiated. I shouldn't have to add complexity to my code to say "In the AWS case, the variable is called "region", but in the OpenStack case, it's called "openstack_region".

This pull request is aimed at fixing this issue.