🐛 Add zk inventory group var into broker role

[rmarting]

This PR fixes an issue coming from the #44. The amq_streams_zookeeper_inventory_group variable must be part also the amq_streams_broker role just because the server.properties.j2 template uses it to set up the connection to the zookeeper cluster.

The issue was:

TASK [amq_streams_common : Ensure amq_streams_broker configuration is deployed.] **********************************************************
An exception occurred during task execution. To see the full traceback, use -vvv. The error was: ansible.errors.AnsibleUndefinedVariable: 'amq_streams_zookeeper_inventory_group' is undefined
fatal: [f38mw01]: FAILED! => {"changed": false, "msg": "AnsibleUndefinedVariable: 'amq_streams_zookeeper_inventory_group' is undefined"}
An exception occurred during task execution. To see the full traceback, use -vvv. The error was: ansible.errors.AnsibleUndefinedVariable: 'amq_streams_zookeeper_inventory_group' is undefined

@rpelisse Can you review it? Maybe there is another way to declare these variables to identify the group vars.