server.properties: do existence check on ‘kafka_listen_address’

was getting this error prior to fix: 

```
TASK: [kafka | create server.properties] ************************************** 
<192.168.33.19> ESTABLISH CONNECTION FOR USER: vagrant
<192.168.33.19> EXEC ssh -C -tt -vvv -o ForwardAgent=yes -o ControlMaster=auto -o ControlPersist=60s -o ControlPath=/Users/tom/.ansible/cp/ansible-ssh-%h-%p-%r -o StrictHostKeyChecking=no -o IdentityFile="/Users/tom/.vagrant.d/insecure_private_key" -o KbdInteractiveAuthentication=no -o PreferredAuthentications=gssapi-with-mic,gssapi-keyex,hostbased,publickey -o PasswordAuthentication=no -o User=vagrant -o ConnectTimeout=10 192.168.33.19 /bin/sh -c 'mkdir -p $HOME/.ansible/tmp/ansible-tmp-1413498244.14-110580405837030 && chmod a+rx $HOME/.ansible/tmp/ansible-tmp-1413498244.14-110580405837030 && echo $HOME/.ansible/tmp/ansible-tmp-1413498244.14-110580405837030'
fatal: [kafka1] => {'msg': "AnsibleUndefinedVariable: One or more undefined variables: 'kafka_listen_address' is undefined", 'failed': True}
fatal: [kafka1] => {'msg': "AnsibleUndefinedVariable: One or more undefined variables: 'kafka_listen_address' is undefined", 'failed': True}

FATAL: all hosts have already failed -- aborting

PLAY RECAP ******************************************************************** 
           to retry, use: --limit @/Users/tom/servers.retry

kafka1                     : ok=47   changed=37   unreachable=1    failed=0   

Ansible failed to complete successfully. Any error output should be
visible above. Please fix these errors and try again.
```