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.go

@@ -0,0 +1,28 @@
+package user
+
+import (
+	"github.com/spf13/afero"
+	"github.com/willdonnelly/passwd"
+)
+
+// An Entry contains all the fields for a specific user. Re-exported to allow other module to only import this passwd module.
+type Entries map[string]passwd.Entry
+
+type Passwd struct{}
+
+// Parse opens the '/etc/passwd' file and parses it into a map from usernames to Entries.
+func (p Passwd) Parse(fs afero.Fs) (Entries, error) {
+	return p.parseFile(fs, "/etc/passwd")
+}
+
+// parseFile opens the file and parses it into a map from usernames to Entries.
+func (p Passwd) parseFile(fs afero.Fs, path string) (Entries, error) {
+	file, err := fs.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	entries, err := passwd.ParseReader(file)
+	return Entries(entries), err
+}