Add back the external_url in the prometheus block

[surajssd]

Remove this overriding of the ingress.host as it is done here:

lokomotive/pkg/components/prometheus-operator/template.go

Lines 119 to 121 in df2951c

	{{ if .Prometheus.Ingress }}
	externalUrl: https://{{.Prometheus.Ingress.Host}}
	{{ end }}

Add back the dedicated external_url to the prometheus block, like it is done for the alertmanager.

[johananl]

@surajssd can this be closed before the next sprint?

[surajssd]

@johananl yep the PR is open, PTAL #964