Set prom_metrics_display_url automatically

[briend]

Previously in the Helm Chart, we were setting TERASLICE_DISPLAY_URL env variable manually to whatever we configured the ingress for. It would be convenient if prom_metrics_display_url for the new internal metrics server was configured automatically for us, and include a trailing / to preserve the behavior of the old exporter.

Basically some logic in the templating that would do something like this:

• If ingress is enabled, pick the first host in the hosts array and set prom_metrics_display_url to that, but also set http(s) appropriately, and add a trailing / if necessary. ex https://teraslice.local/
• Otherwise, if ingress is not enabled, leave prom_metrics_display_url unset.
• if prom_metrics_display_url is set by user, use that as-is without any modification.