Description of problem:

Rolling update fails for task '[select a running monitor if multiple monitors]'
 
TASK [select a running monitor if multiple monitors] ************************************************************************************************************************
Monday 03 December 2018  06:16:59 +0000 (0:00:00.135)       0:02:33.577 ******* 
fatal: [magna023]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: No last item, sequence was empty.\n\nThe error appears to have been in '/usr/share/ceph-ansible/rolling_update.yml': line 155, column 7, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n    - name: select a running monitor if multiple monitors\n      ^ here\n"}
 

Version-Release number of selected component (if applicable):
ceph-ansible-3.2.0-0.1.rc6.el7cp.noarch


How reproducible:
Always 

Steps to Reproduce:
1. Do rolling update via ansible
2. Playbook fails at task [select a running monitor if multiple monitors] 



Additional info:

Here we have used only one mon 'non-containerised' cluster for upgrade. 

So, as upgrade of a single monitor is not recommended, the play-book should ideally fail at a task say ("fail when single non containerized monitor") in the same way it errors out while rolling-update in a containerized deployment.