Prometheus Operator docs: Add grafana_ingress_host

This commit adds documentation of the variable `grafana_ingress_host`
which allows user to expose Grafana deployed by Prometheus Operator.

Signed-off-by: Suraj Deshmukh <suraj@kinvolk.io>

docs/configuration-reference/components/prometheus-operator.md

@@ -29,8 +29,10 @@ Prometheus Operator component configuration example:
 
 ```tf
 component "prometheus-operator" {
-  namespace              = "monitoring"
+  namespace = "monitoring"
+
   grafana_admin_password = "foobar"
+  grafana_ingress_host   = "grafana.mydomain.net"
 
   etcd_endpoints = [
     "10.88.181.1",
@@ -86,6 +88,7 @@ Example:
 |--------	|--------------|:-------:|:--------:|
 | `namespace` | Namespace to deploy the Prometheus Operator. | - | true |
 | `grafana_admin_password` | Password for `admin` user in Grafana.  | - | true |
+| `grafana_ingress_host` | Ingress URL Host to expose Grafana over internet. **NOTE:** User will have to create a DNS entry for this against Packet EIP manually if running on Packet.  | - | false |
 | `etcd_endpoints` | List of endpoints where etcd can be reachable from Kubernetes. | [] | false |
 | `prometheus_operator_node_selector` | Node selector to specify nodes where the Prometheus Operator pods should be deployed. | {} | false |
 | `prometheus_metrics_retention` | Time duration Prometheus shall retain data for. Must match the regular expression `[0-9]+(ms\|s\|m\|h\|d\|w\|y)` (milliseconds, seconds, minutes, hours, days, weeks and years). | `10d` | false |