Simplify Mockserver expectation

[jsmolar]

This PR adds some functionality and simplifies Mockserver

• removes the unnecessary path argument for the create_expectation method. The expectation_id is used to create a path for expectation. It is possible to create expectations with scope function.
• adds a new method that creates an expectation for templated responses: https://www.mock-server.com/mock_server/response_templates.html
• an instance of mockserver now stores live expectations

[pehala]

I would be happier if the new methods are added only when needed (=in the PR that requires them), but I will approve this nonetheless.