Allow customizing which URL to use when running the performance test

[linde9821]

When importing a specification the tool seems to always use the URL set in endpoint_url when constructing an OpenAPIPerf instance. Meaning it overrides the URL specified in the openapi description.

This is an undesired behavior when only hosting the file on the provided endpoint_url.
To my knowledge, your API doesn't offer the possibility to change it manually. (except by customizing the test json)