option to set KeyManager memory in spire server (#444)

charts/spire/charts/spire-server/templates/configmap.yaml

@@ -1,5 +1,6 @@
 {{- define "spire-server.yaml-config" -}}
 {{- $upstreamAuthorityUsed := 0 }}
+{{- $keyManagerUsed := 0 }}
 {{- $root := . }}
 server:
   bind_address: "0.0.0.0"
@@ -47,10 +48,28 @@ plugins:
   {{- end }}
   {{- end }}
 
+  {{- with .Values.keyManager.disk }}
+  {{- if eq (.enabled | toString) "true" }}
+  {{- $keyManagerUsed = add1 $keyManagerUsed }}
   KeyManager:
     - disk:
         plugin_data:
           keys_path: "/run/spire/data/keys.json"
+  {{- end }}
+  {{- end }}
+
+  {{- with .Values.keyManager.memory }}
+  {{- if eq (.enabled | toString) "true" }}
+  {{- $keyManagerUsed = add1 $keyManagerUsed }}
+  KeyManager:
+    - memory:
+        plugin_data:
+  {{- end }}
+  {{- end }}
+
+{{- if gt $keyManagerUsed 1 }}
+{{- fail "You can only enable a single KeyManager" }}
+{{- end }}
 
   Notifier:
     - k8sbundle: