drop-in: add support for drop-ins

This allows a list of drop-ins for a unit to be declared inline within a
cloud-config. For example:

  #cloud-config
  coreos:
    units:
      - name: docker.service
        drop-ins:
          - name: 50-insecure-registry.conf
            content: |
              [Service]
              Environment=DOCKER_OPTS='--insecure-registry="10.0.1.0/24"'

config/config_test.go

@@ -335,26 +335,6 @@ func TestCloudConfigSerializationHeader(t *testing.T) {
 	}
 }
 
-// TestDropInIgnored asserts that users are unable to set DropIn=True on units
-func TestDropInIgnored(t *testing.T) {
-	contents := `
-coreos:
-  units:
-    - name: test
-      dropin: true
-`
-	cfg, err := NewCloudConfig(contents)
-	if err != nil || len(cfg.Coreos.Units) != 1 {
-		t.Fatalf("Encountered unexpected error: %v", err)
-	}
-	if len(cfg.Coreos.Units) != 1 || cfg.Coreos.Units[0].Name != "test" {
-		t.Fatalf("Expected 1 unit, but got %d: %v", len(cfg.Coreos.Units), cfg.Coreos.Units)
-	}
-	if cfg.Coreos.Units[0].DropIn {
-		t.Errorf("dropin option on unit in cloud-config was not ignored!")
-	}
-}
-
 func TestCloudConfigUsers(t *testing.T) {
 	contents := `
 users:

config/unit.go

@@ -17,15 +17,16 @@
 package config
 
 type Unit struct {
-	Name    string `yaml:"name"`
-	Mask    bool   `yaml:"mask"`
-	Enable  bool   `yaml:"enable"`
-	Runtime bool   `yaml:"runtime"`
-	Content string `yaml:"content"`
-	Command string `yaml:"command" valid:"start,stop,restart,reload,try-restart,reload-or-restart,reload-or-try-restart"`
-
-	// For drop-in units, a cloudinit.conf is generated.
-	// This is currently unbound in YAML (and hence unsettable in cloud-config files)
-	// until the correct behaviour for multiple drop-in units is determined.
-	DropIn bool `yaml:"-"`
+	Name    string       `yaml:"name"`
+	Mask    bool         `yaml:"mask"`
+	Enable  bool         `yaml:"enable"`
+	Runtime bool         `yaml:"runtime"`
+	Content string       `yaml:"content"`
+	Command string       `yaml:"command" valid:"start,stop,restart,reload,try-restart,reload-or-restart,reload-or-try-restart"`
+	DropIns []UnitDropIn `yaml:"drop_ins"`
+}
+
+type UnitDropIn struct {
+	Name    string `yaml:"name"`
+	Content string `yaml:"content"`
 }