Allow custom values for Self Hosted Clusters (Slugs, Titles, Region) (#4121)

# What this PR does
Allows for environment variables to be set on Grafana OnCall Engine for
Self Hosted users, giving them the ability to set values for Stack Slug,
Org Slug/Title, Region & Cluster Slugs.

This will mean then using the Grafana OnCall App, when adding multiple
OSS Stacks, you can set the correct value of 'stack_slug' so you can
differentiate between the stacks in the App.

## Which issue(s) this PR closes

Closes [#4119](https://github.com/grafana/oncall/issues/4119)

## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

Co-authored-by: Joey Orlando <joey.orlando@grafana.com>

engine/settings/base.py

@@ -736,13 +736,13 @@ GRAFANA_CLOUD_AUTH_API_SYSTEM_TOKEN = os.environ.get("GRAFANA_CLOUD_AUTH_API_SYS
 
 SELF_HOSTED_SETTINGS = {
     "STACK_ID": 5,
-    "STACK_SLUG": "self_hosted_stack",
+    "STACK_SLUG": os.environ.get("SELF_HOSTED_STACK_SLUG", "self_hosted_stack"),
     "ORG_ID": 100,
-    "ORG_SLUG": "self_hosted_org",
-    "ORG_TITLE": "Self-Hosted Organization",
-    "REGION_SLUG": "self_hosted_region",
+    "ORG_SLUG": os.environ.get("SELF_HOSTED_ORG_SLUG", "self_hosted_org"),
+    "ORG_TITLE": os.environ.get("SELF_HOSTED_ORG_TITLE", "Self-Hosted Organization"),
+    "REGION_SLUG": os.environ.get("SELF_HOSTED_REGION_SLUG", "self_hosted_region"),
     "GRAFANA_API_URL": os.environ.get("GRAFANA_API_URL", default=None),
-    "CLUSTER_SLUG": "self_hosted_cluster",
+    "CLUSTER_SLUG": os.environ.get("SELF_HOSTED_CLUSTER_SLUG", "self_hosted_cluster"),
 }
 
 GRAFANA_INCIDENT_STATIC_API_KEY = os.environ.get("GRAFANA_INCIDENT_STATIC_API_KEY", None)