pom.xml: make Maven integration plugin optional

[hashar]

maven-plugin has been added as a dependency with 30ca891 along with jenkins-war 2.3 and jenkins-core 2.3 and I guess it was no more bundled in Jenkins as a result.

The only usage is in hudson.plugins.im.bot.HealthCommandTest, otherwhise instant messaging does not depend on the plugin, it merely supports it.

Make the Maven plugin optional, this allows administrators to remove the plugin when it is otherwise not used.

Note: given the Maven plugin is only needed for testing the integration, my guess is that it should also be marked with <scope>test</scope>?

Testing done

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue