Move processing of cli args and config file to separate module

[tlestang]

The configuration for cats is specified by both a config file and cli arguments. Values provided at the command line often override the values provided in the config file. It means there is a bit of work to be done in order to distil all that information into a fixed runtime configuration before cats can start making a request to a CI service. Namely, cats needs

• The location
• The APIInterface instance that prescribes how to contact the CI service
• The job duration

This PR moves the processing required to set these three things (the cats runtime configuration) into a separate module configure that provides a single configure function as an entry point. Benefits are:

• Less code in __init__.py which is starting get big
• Easier to add unit tests around the processing of config file and cli arguments

[tlestang]

Thanks for the pointers, I never had a chance to use a mock before. It does look a lot better.

[tlestang]

This is going in, thanks @abhidg and @sadielbartholomew for the review!