feat(unit): Allow user to control enabling units

Fix #69 - A user may provide an `enable` attribute of a unit in their
cloud config document. If true, coreos-cloudinit will instruct systemd
to enable the associated unit. If false, the unit will not be enabled.

Fix #71 - The default enable behavior has been changed from on to off.

Documentation/cloud-config.md

@@ -87,6 +87,7 @@ Arbitrary systemd units may be provided in the `coreos.units` attribute.
 
 - **name**: String representing unit's name. Required.
 - **runtime**: Boolean indicating whether or not to persist the unit across reboots. This is analagous to the `--runtime` argument to `systemd enable`. Default value is false.
+- **enable**: Boolean indicating whether or not to handle the [Install] section of the unit file. This is similar to running `systemctl enable <name>`. Default value is false.
 - **content**: Plaintext string representing entire unit file. If no value is provided, the unit is assumed to exist already.
 - **command**: Command to execute on unit: start, stop, reload, restart, try-restart, reload-or-restart, reload-or-try-restart. Default value is restart.
 

initialize/config.go

@@ -140,14 +140,16 @@ func Apply(cfg CloudConfig, env *Environment) error {
 				}
 				log.Printf("Placed unit %s at %s", unit.Name, dst)
 
-				if unit.Group() != "network" {
-					log.Printf("Enabling unit file %s", dst)
-					if err := system.EnableUnitFile(dst, unit.Runtime); err != nil {
-						return err
+				if unit.Enable {
+					if unit.Group() != "network" {
+						log.Printf("Enabling unit file %s", dst)
+						if err := system.EnableUnitFile(dst, unit.Runtime); err != nil {
+							return err
+						}
+						log.Printf("Enabled unit %s", unit.Name)
+					} else {
+						log.Printf("Skipping enable for network-like unit %s", unit.Name)
 					}
-					log.Printf("Enabled unit %s", unit.Name)
-				} else {
-					log.Printf("Skipping enable for network-like unit %s", unit.Name)
 				}
 			}
 

system/systemd.go

@@ -19,6 +19,7 @@ const fakeMachineID = "42000000000000000000000000000042"
 
 type Unit struct {
 	Name    string
+	Enable  bool
 	Runtime bool
 	Content string
 	Command string