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

coordinator/core/ssh.go

@@ -0,0 +1,21 @@
+package core
+
+import (
+	"context"
+
+	"github.com/edgelesssys/constellation/internal/deploy/ssh"
+)
+
+// CreateSSHUsers creates UNIX users with respective SSH access on the system the coordinator is running on when defined in the config.
+func (c *Core) CreateSSHUsers(sshUserKeys []ssh.UserKey) error {
+	sshAccess := ssh.NewSSHAccess(c.linuxUserManager)
+	ctx := context.Background()
+
+	for _, pair := range sshUserKeys {
+		if err := sshAccess.DeploySSHAuthorizedKey(ctx, pair); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}