metropolis: implement cluster configuration

This adds a cluster configuration to Metropolis. We'll be keeping any
non-node-specific options there. The config is stored in etcd by the
curator.

An initial cluster configuration can be specified when bootstrapping a
cluster. By design the configuration is then immutable by default, but
we might add some purpose-specific management API calls to change some
values if needed.

We initialize the cluster configuration with a setting for node TPM
policy, 'TPMMode'. It's currently populated on cluster bootstrap, but
not used otherwise. That will come in a follow-up CR.

Change-Id: I44ddcd099c9ae68c20519c77e3fa77c894cf5a20
Reviewed-on: https://review.monogon.dev/c/monogon/+/1494
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
Tested-by: Jenkins CI
diff --git a/metropolis/proto/api/management.proto b/metropolis/proto/api/management.proto
index 5b66311..5560455 100644
--- a/metropolis/proto/api/management.proto
+++ b/metropolis/proto/api/management.proto
@@ -90,6 +90,8 @@
 
     // ca_certificate is the x509 DER encoded CA certificate of the cluster.
     bytes ca_certificate = 2;
+
+    metropolis.proto.common.ClusterConfiguration cluster_configuration = 3;
 }
 
 message GetNodesRequest {