fix: use non-fqdn url for grafana to support non-standard cluster names

[weisdd]

Description

Currently, grafana-operator uses FQDN names for its calls to grafana API (e.g. http://grafana.monitoring.svc.cluster.local:3000), which is incompatible with custom cluster names.
For internal DNS requests, there's no need to rely on FQDN, <svc-name>.<namespace>:<port> format is perfectly fine.
The PR addresses that.

Additional changes:

• Some code style issues are automatically fixed by gofumpt;
• Fixed staticcheck complaint about error formatting.

Relevant issues/tickets

Fixes: #592

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• This change requires a documentation update
• I have added tests that prove my fix is effective or that my feature works
• I have added a test case that will be used to verify my changes
• Verified independently on a cluster by reviewer

Verification steps

1. Deploy grafana instance with any spec
2. Make sure operator is able to communicate with grafana:

• For clusters with standard names, operator should work just as before;
• For clusters with a custom name, operator should start working.

[HubertStefanski]

Works fine with preferService: False but I get this error on OpenShift when set to True

	running periodic dashboard resync
2022-07-19T12:27:11.335+0100	ERROR	failed to get or create namespace folder for dashboard	{"folder": "hstefans", "dashboard": "", "error": "Get \"http://admin:***@grafana-service.hstefans:3000/api/folders\": dial tcp: lookup grafana-service.hstefans: no such host"}
github.com/go-logr/zapr.(*zapLogger).Error
	/home/hstefans/go/src/github.com/grafana-operator/grafana-operator/vendor/github.com/go-logr/zapr/zapr.go:132
sigs.k8s.io/controller-runtime/pkg/log.(*DelegatingLogger).Error
	/home/hstefans/go/src/github.com/grafana-operator/grafana-operator/vendor/sigs.k8s.io/controller-runtime/pkg/log/deleg.go:144
github.com/grafana-operator/grafana-operator/v4/controllers/grafanadashboard.(*GrafanaDashboardReconciler).reconcileDashboards
	/home/hstefans/go/src/github.com/grafana-operator/grafana-operator/controllers/grafanadashboard/grafanadashboard_controller.go:294
github.com/grafana-operator/grafana-operator/v4/controllers/grafanadashboard.(*GrafanaDashboardReconciler).Reconcile
	/home/hstefans/go/src/github.com/grafana-operator/grafana-operator/controllers/grafanadashboard/grafanadashboard_controller.go:101
github.com/grafana-operator/grafana-operator/v4/controllers/grafanadashboard.SetupWithManager.func1
	/home/hstefans/go/src/github.com/grafana-operator/grafana-operator/controllers/grafanadashboard/grafanadashboard_controller.go:184
github.com/grafana-operator/grafana-operator/v4/controllers/grafanadashboard.SetupWithManager.func2
	/home/hstefans/go/src/github.com/grafana-operator/grafana-operator/controllers/grafanadashboard/grafanadashboard_controller.go:193
2022-07-19T12:27:11.335+0100	ERROR	error updating dashboard	{"name": "simple-dashboard", "namespace": "hstefans", "error": "Get \"http://admin:***@grafana-service.hstefans:3000/api/folders\": dial tcp: lookup grafana-service.hstefans: no such host"}

the service does exist

I'm also not 100% sure this is an issue with your PR specifically.

code looks good other than that

[HubertStefanski]

Approving! I managed to verify this on my Openshift cluster, I must have wrongly configured something with the previous attempt, LGTM