Bump amazon broker version for multimachine scenario

[pmacius]

When trying to update to 5.18.7 so we have the specific version hardcoded:

"An error occurred (BadRequestException) when calling the CreateBroker operation: Broker engine version [5.18.7] is invalid for broker engine type [ACTIVEMQ]. Valid values: [5.18]."

After setting it to 5.18 it also requires:

"An error occurred (BadRequestException) when calling the CreateBroker operation: Brokers on [ActiveMQ] version [5.18] must have [autoMinorVersionUpgrade] set to [true]."