Description of problem:
Error message when setting osd_scenario is missed. Not defining osd_scenario must be handled and user understandable error must be displayed.

Version-Release number of selected component (if applicable):
ceph-ansible-3.2.0-0.1.rc4.el7cp.noarch

How reproducible:
Always

Steps to Reproduce:
1. Configure ceph-ansible to initialise a cluster and don't set any value to osd_scenario.
2. Run playbook

Actual results:
TASK [ceph-validate : validate provided configuration] 

.........
Notario Failure: key did not match schema  (required key in data is missing: <function validate_osd_scenarios at 0x7fa0f09bf500>)
The full traceback is:
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/ansible/executor/task_executor.py", line 139, in run
    res = self._execute()
  File "/usr/lib/python2.7/site-packages/ansible/executor/task_executor.py", line 584, in _execute
    result = self._handler.run(task_vars=variables)
  File "/usr/share/ceph-ansible/plugins/actions/validate.py", line 100, in run
    msg = "[{}] Validation failed for variable: {}".format(host, error.path[0])

IndexError: list index out of range

fatal: [node07]: FAILED! => {
    "msg": "Unexpected failure during module execution.", 
    "stdout": ""
}

.......

 [ERROR]: [magna032] Validation failed for variable: item[0]

 [ERROR]: [magna032] Reason: -> item[0] key did not match 'osd_scenario' (required item in schema is missing: osd_scenario)

fatal: [magna032]: FAILED! => {
    "changed": false, 

"msg": "[node32] Validation failed for variable: item[0]\n[magna032] Reason: -> item[0] key did not match 'osd_scenario' (required item in schema is missing: osd_scenario)\n", 
    "stderr_lines": [
        "[node32] Validation failed for variable: item[0]"


Expected results:
Appropriate error message must be displayed when a variable is not set

Additional info:
(Error message for node24 and node7 were same)

$ ssh ubuntu@node07 cat /etc/ansible/hosts
[mons]
node07
node24
node32

[osds]
node24 osd_auto_discovery='true' devices="['/dev/sdb']" 
node07 dmcrypt='true' devices="['/dev/sdb']"  
node32 devices="['/dev/sdb','/dev/sdc']" 

[mgrs]
node07

$ cat /usr/share/ceph-ansible/group_vars/all.yml| egrep -v ^# | grep -v ^$
---
dummy:
fetch_directory: ~/ceph-ansible-keys
ceph_origin: distro
ceph_repository: rhcs
ceph_rhcs_version: 3
ceph_docker_image: "rhceph"
ceph_docker_image_tag: "ceph-3.2-rhel-7-containers-candidate-38068-20181129153049"
ceph_docker_registry: "brew-pulp-docker01........"
monitor_interface: eno1
public_network: 10.8.128.0/21
containerized_deployement: True