[BUG] The program crashes if the image name does not contain colon

**What version of operator are you using?**
commit 561cf47d783c368fd8795acb82a5a39099a35984 (HEAD -> master)

**What operating system and processor architecture are you using (`kubectl version`)?**
Ubuntu. 20.04

kubectl version Output

$ kubectl version

Client Version: version.Info{Major:"1", Minor:"24", GitVersion:"v1.24.0", GitCommit:"4ce5a8954017644c5420bae81d72b09b735c21f0", GitTreeState:"clean", BuildDate:"2022-05-03T13:46:05Z", GoVersion:"go1.18.1", Compiler:"gc", Platform:"darwin/amd64"}

Kustomize Version: v4.5.4

Server Version: version.Info{Major:"1", Minor:"22", GitVersion:"v1.22.9", GitCommit:"6df4433e288edc9c40c2e344eb336f63fad45cd2", GitTreeState:"clean", BuildDate:"2022-05-19T19:53:08Z", GoVersion:"go1.16.15", Compiler:"gc", Platform:"linux/amd64"}

WARNING: version difference between client (1.24) and server (1.22) exceeds the supported minor version skew of +/-1

**What did you do?**

We found that crdb-operator will crash if we do not supply version for cockroachdb image. We supplied `cockroachdb/cockroach` for the `spec.image` field, and the operator crashes at [this line](https://github.com/cockroachdb/cockroach-operator/blob/dda0a6308b0f1c64d41e0851a5ef8cb60cd98a37/pkg/resource/cluster.go#L350)

### Reproduce

We first applied the operator yaml file and crd yaml file to deploy the operator.
Then we applied the following cr.yaml file to deploy the cockroachdb cluster:
by using `kubectl apply -f cr.yaml -n cockroach-operator-system`

**cr.yaml:**
```yaml
apiVersion: crdb.cockroachlabs.com/v1alpha1
kind: CrdbCluster
metadata:
name: test-cluster
spec:
additionalLabels:
crdb: is-cool
dataStore:
pvc:
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 1Gi
volumeMode: Filesystem
image:
name: cockroachdb/cockroach
nodes: 3
resources:
limits:
cpu: 2
memory: 2Gi
requests:
cpu: 100m
memory: 1Gi
tlsEnabled: true

```

**What did you see?**

The operator crashes.

**Possible root cause**
It seems that the cockroach operator has assumed that there will always be a colon in the image name, as [this line](https://github.com/cockroachdb/cockroach-operator/blob/dda0a6308b0f1c64d41e0851a5ef8cb60cd98a37/pkg/resource/cluster.go#L350) suggests. There is no code preventing the program from crashing if an image name without a colon is provided. And if the image name does not have a colon, there will be an "index going out of range" error.

This is a bug, since crashing behavior is considered as bad behavior. There should be a sanity check to examine the validation of the image name, at least not let the program crash.

Start in pkg/resource/cluster.go around line 350, where the issue reports the crash for an image name without a colon. Reproduce it with the provided cr.yaml and kubectl apply command, then verify the image is handled without an index-out-of-range crash and that invalid input receives a controlled response.

Written by the indexing model from the issue text.