Step 8 of "Chapter 3. Adding a Remote Console User" in A-MQ 6.1 Installation Guide states that activemq.jmx.user and activemq.jmx.password needs to be set in system.properties file to match the values you just defined in the etc/users.properties file. But it's actually not needed.

https://access.redhat.com/documentation/en-US/Red_Hat_JBoss_A-MQ/6.1/html-single/Installation_Guide/index.html#InstallAddUser