[Java Spring Boot] - Issue with circleci CI tests for spring-mvc

Description

There seems to be an issue with the circle CI tests for spring-mvc. On multiple pull requests I've observed an issue where the test step mvn -q verify -Psamples fails.

https://circleci.com/gh/swagger-api/swagger-codegen/4228?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link
https://circleci.com/gh/swagger-api/swagger-codegen/4227?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link
https://circleci.com/gh/swagger-api/swagger-codegen/4173?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link
https://circleci.com/gh/swagger-api/swagger-codegen/4054?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link
...

Error seems to be:

[ERROR] Failed to execute goal org.eclipse.jetty:jetty-maven-plugin:9.2.15.v20160210:start (start-jetty) on project swagger-spring-mvc-j8-localdatetime: Failure: ShutdownMonitorThread already started -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
[ERROR]
[ERROR] After correcting the problems, you can resume the build with the command
[ERROR] mvn -rf :swagger-spring-mvc-j8-localdatetime

Swagger-codegen version

2.4.0-SNAPSHOT

Swagger declaration file content or url

N/A

Command line used for generation

mvn -q verify -Psamples

Steps to reproduce

1. Submit PR to master.

Related issues/PRs

Suggest a fix/enhancement

Start by reproducing the CircleCI failure with mvn -q verify -Psamples, focusing on the spring-mvc sample that reports ShutdownMonitorThread already started. Review the Maven verification setup and the swagger-spring-mvc-j8-localdatetime sample named in the error. Done means the samples verification completes successfully without the Jetty shutdown-monitor error.