Authentication and Service Endpoint Retrieval; Addition of Environment Variable Defaults to Client Library

Authentication and service endpoint retrieval now works with the integration environment using both keep and the client library. Adding the variables defined in the newly added clientrc file to your environment allows for running the client on the integration environment without the need to provide any arguments.

I moved the function in keep that looked up environment variables to the client library, but keep still uses it to be able to display the default values in help messages.

I also improved the docstrings in client.py.

Also, there's one thing I'm not completely sure of. Which should take precedence: The service endpoint defined as an environment variable, or the service endpoint retrieved from Keystone? An endpoint explicitly passed in takes precedence over both, and right now the environment value takes precedence over the endpoint from keystone. This makes sense to me since otherwise the environment defined endpoint wouldn't serve much purpose and could result in false expectations.