config/system: add shell user attribute

This adds support for specifying the login shell of created users.

config/config_test.go

@@ -374,6 +374,7 @@ users:
     no_user_group: true
     system: y
     no_log_init: True
+    shell: /bin/sh
 `
 	cfg, err := NewCloudConfig(contents)
 	if err != nil {
@@ -441,6 +442,10 @@ users:
 	if !user.NoLogInit {
 		t.Errorf("Failed to parse no_log_init field")
 	}
+
+	if user.Shell != "/bin/sh" {
+		t.Errorf("Failed to parse shell field, got %q", user.Shell)
+	}
 }
 
 func TestCloudConfigUsersGithubUser(t *testing.T) {

config/user.go

@@ -29,4 +29,5 @@ type User struct {
 	NoUserGroup          bool     `yaml:"no_user_group"`
 	System               bool     `yaml:"system"`
 	NoLogInit            bool     `yaml:"no_log_init"`
+	Shell                string   `yaml:"shell"`
 }