Add support for configurable OSD DB ratios

The default of 0.05 (5%) is likely ideal in the initial implementation,
but allow this to be set explicitly for maximum flexibility in
space-constrained or performance-critical use-cases.

docs/manuals/swagger.json

@@ -5026,6 +5026,13 @@
                         "name": "ext_db",
                         "required": false,
                         "type": "boolean"
+                    },
+                    {
+                        "description": "Decimal ratio of total OSD size for the external OSD DB LV device, default 0.05 (5%)",
+                        "in": "query",
+                        "name": "ext_db_ratio",
+                        "required": false,
+                        "type": "float"
                     }
                 ],
                 "responses": {