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_test.go

@@ -0,0 +1,36 @@
+package ssh
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestToAndFromProtoSlice(t *testing.T) {
+	assert := assert.New(t)
+
+	DemoSSHUser1 := UserKey{
+		Username:  "test-user-2",
+		PublicKey: "ssh-rsa abcdefg",
+	}
+
+	DemoSSHUser2 := UserKey{
+		Username:  "test-user-2",
+		PublicKey: "ssh-rsa hijklmnop",
+	}
+
+	// Input usually consists of pointers (from config parsing)
+	DemoSSHUsersPointers := make([]*UserKey, 0)
+	DemoSSHUsersPointers = append(DemoSSHUsersPointers, &DemoSSHUser1)
+	DemoSSHUsersPointers = append(DemoSSHUsersPointers, &DemoSSHUser2)
+
+	// Expected output usually does not consist of pointers
+	DemoSSHUsersNoPointers := make([]UserKey, 0)
+	DemoSSHUsersNoPointers = append(DemoSSHUsersNoPointers, DemoSSHUser1)
+	DemoSSHUsersNoPointers = append(DemoSSHUsersNoPointers, DemoSSHUser2)
+
+	ToProtoArray := ToProtoSlice(DemoSSHUsersPointers)
+	FromProtoArray := FromProtoSlice(ToProtoArray)
+
+	assert.Equal(DemoSSHUsersNoPointers, FromProtoArray)
+}