Use random port in spring-boot tests

This commit updates the test suite of the spring-boot module to use random ports instead of hard coding `8080` or `8081`. When this is not possible and a different value is required, `SocketUtils` is used to find
a random ports in the range above 40000.

A new utility class has been created. `WebTestUtils` has only one method for now and creates a local url for a given embedded container.

Partly fixes gh-337