Previously, Rook would fail to create monitors for a `CephCluster` spec that
required them to be located in distinct failure domains:
```yaml
spec:
mon:
count: 3
failureDomainLabel: topology.kubernetes.io/zone
zones:
- eu-central-1a
- eu-central-1b
- eu-central-1c
```
This was due to the fact that Rook was mistakenly getting the
`failureDomainLabel` from `spec.mon.stretchCluster.failureDomainLabel`.
This commit ensures `spec.mon.failureDomainLabel` is being used instead, which
fixes the cluster deployment for the spec above.
Signed-off-by: Benoît Knecht <bknecht@protonmail.ch>