XD-2097 Shutdown containers from admin

- Enable `shutdown` endpoint for containers
 - Keep the `mangementContextPath` the same for both admin/container
- ContainerRegistrar stores the container's port as container attribute
  - If the container uses local port `0` then the port is known only when
the `EmbeddedServletContainer` is initialized
    - Add `update` method in `ContainerRepository` to update the container data
    - Update container data (attributes) along with container port info
- Add REST endpoint that shuts down container by `containerId`
  - Get container host and port from container attributes
  - Use `RestTemplate` to send `POST` request to shutdown the container
  - Throw `NoSuchContainerException` if container is not found
  - Add test
By setting `endpoints.shutdown.enabled` to true in `application.yml` we enable Boot's `ShutdownEndpoint` which is a sensitive (secure) endpoint (though we don't have any security constraints added yet). When this endpoint is invoked, it closes the application context (in our case ContainerServer's parent context and subsequently the child contexts). Please see: https://github.com/spring-projects/spring-boot/blob/master/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/ShutdownEndpoint.java