Fix hosts field in rolling_update playbook when mds are processed

Fix hosts field in rolling_update playbook when mds are processed

In the OSP context, during the rolling update the playbook fails
with the following error:

'''
ERROR! The field 'hosts' has an invalid value, which includes an
undefined variable. The error was: list object has no element 0
'''

This PR just change the hosts field providing a valid mons group
value.

Closes: https://bugzilla.redhat.com/1876803
Signed-off-by: Francesco Pantano <fpantano@redhat.com>