Support tomcat users

@bmuschko As we discussed in #107, supporting configure basic authentication user

closes #107
Can you please use the method `AvailablePortFinder.createPrivate()` here as used in [AbstractIntegrationTest](https://github.com/nykolaslima/gradle-tomcat-plugin/blob/support_tomcat_users/plugin/src/integTest/groovy/org/gradle/api/plugins/tomcat/AbstractIntegrationTest.groovy).