take-over: fix bug when trying to override variable

take-over: fix bug when trying to override variable

A customer has been facing an issue when trying to override
`monitor_interface` in inventory host file.
In his use case, all nodes had the same interface for
`monitor_interface` name except one. Therefore, they tried to override
this variable for that node in the inventory host file but the
take-over-existing-cluster playbook was failing when trying to generate
the new ceph.conf file because of undefined variable.

Typical error:

```
fatal: [srvcto103cnodep01]: FAILED! => {"failed": true, "msg": "'dict object' has no attribute u'ansible_bond0.15'"}
```

Including variables like this `include_vars: group_vars/all.yml` prevent
us from overriding anything in inventory host file because it
overwrites everything you would have defined in inventory.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1575915
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>