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/user/create_user.go

@@ -0,0 +1,25 @@
+package user
+
+import (
+	"context"
+	"fmt"
+	"os/exec"
+)
+
+type Unix struct{}
+
+// reference: https://man7.org/linux/man-pages/man8/useradd.8.html#EXIT_VALUES
+const exitCodeUsernameAlreadyInUse = 9
+
+// CreateUser creates a new user with sudo access. Returns successfully if creation succeeds or user existed already.
+func (u Unix) CreateUser(ctx context.Context, username string) error {
+	cmd := exec.CommandContext(ctx, "useradd", "-m", "-G", "wheel,sudo", username)
+	if err := cmd.Run(); err != nil {
+		// do not fail if user already exists
+		if exitError, ok := err.(*exec.ExitError); ok && exitError.ExitCode() == exitCodeUsernameAlreadyInUse {
+			return nil
+		}
+		return fmt.Errorf("creating a new user failed: %w", err)
+	}
+	return nil
+}