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/passwd_test.go

@@ -0,0 +1,66 @@
+package user
+
+import (
+	"testing"
+
+	"github.com/spf13/afero"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestParse(t *testing.T) {
+	filename := "/etc/passwd"
+
+	testCases := map[string]struct {
+		passwdContents string
+		createFile     bool
+		wantEntries    Entries
+		wantErr        bool
+	}{
+		"parse works": {
+			passwdContents: "root:x:0:0:root:/root:/bin/bash\n",
+			createFile:     true,
+			wantEntries: Entries{
+				"root": {
+					Pass:  "x",
+					Uid:   "0",
+					Gid:   "0",
+					Gecos: "root",
+					Home:  "/root",
+					Shell: "/bin/bash",
+				},
+			},
+			wantErr: false,
+		},
+		"passwd is corrupt": {
+			passwdContents: "too:few:fields\n",
+			createFile:     true,
+			wantErr:        true,
+		},
+		"file does not exist": {
+			createFile: false,
+			wantErr:    true,
+		},
+	}
+
+	for name, tc := range testCases {
+		t.Run(name, func(t *testing.T) {
+			assert := assert.New(t)
+			require := require.New(t)
+
+			fs := afero.NewMemMapFs()
+			if tc.createFile {
+				assert.NoError(afero.WriteFile(fs, filename, []byte(tc.passwdContents), 0o644))
+			}
+			passwd := Passwd{}
+			entries, err := passwd.Parse(fs)
+
+			if tc.wantErr {
+				assert.Error(err)
+				return
+			}
+			require.NoError(err)
+			assert.Equal(tc.wantEntries, entries)
+		})
+	}
+}