AB#2046 : Add option to create SSH users for the first coordinator upon initialization (#133)

* Move `file`, `ssh` and `user` packages to internal
* Rename `SSHKey` to `(ssh.)UserKey`
* Rename KeyValue / Publickey to PublicKey
* Rename SSH key file from "debugd" to "ssh-keys"
* Add CreateSSHUsers function to Core
* Call CreateSSHUsers users on first control-plane node, when defined in config

Tests:
* Make StubUserCreator add entries to /etc/passwd
* Add NewLinuxUserManagerFake for unit tests
* Add unit tests & adjust existing ones to changes

internal/deploy/ssh/proto.go

@@ -0,0 +1,44 @@
+package ssh
+
+import (
+	"github.com/edgelesssys/constellation/coordinator/pubapi/pubproto"
+)
+
+// FromProtoSlice converts a SSH UserKey definition from pubproto to the Go flavor.
+func FromProtoSlice(input []*pubproto.SSHUserKey) []UserKey {
+	if input == nil {
+		return nil
+	}
+
+	output := make([]UserKey, 0)
+
+	for _, pair := range input {
+		singlePair := UserKey{
+			Username:  pair.Username,
+			PublicKey: pair.PublicKey,
+		}
+
+		output = append(output, singlePair)
+	}
+
+	return output
+}
+
+// ToProtoSlice converts a SSH UserKey definition from Go to pubproto flavor.
+func ToProtoSlice(input []*UserKey) []*pubproto.SSHUserKey {
+	if input == nil {
+		return nil
+	}
+
+	output := make([]*pubproto.SSHUserKey, 0)
+	for _, pair := range input {
+		singlePair := pubproto.SSHUserKey{
+			Username:  pair.Username,
+			PublicKey: pair.PublicKey,
+		}
+
+		output = append(output, &singlePair)
+	}
+
+	return output
+}