Add a way for users to provide bootstrap command

This should allow those who _know what their are doing_ to pass
a custom script to run inside of cloud-init. Anyone looking to
use this will need to defined a shell script that would resemble
`bootstrap.{al2,ubuntu}.sh`. This should cater for any of advanced
use-cases, where additional files must be created on EC2 instances
prior to boot or anything of that nature. Alternative approach is
to use a custom AMI, or propose an eksctl feature, but this is
meant as a stop-gap for testing things out.

pkg/apis/eksctl.io/v1alpha4/types.go

@@ -355,6 +355,8 @@ type NodeGroup struct {
 
 	// +optional
 	IAM *NodeGroupIAM `json:"iam"`
+
+	OverrideBootstrapCommand *string `json:"overrideBootstrapCommand,omitempty"`
 }
 
 // ListOptions returns metav1.ListOptions with label selector for the nodegroup

pkg/nodebootstrap/userdata_al2.go

@@ -40,15 +40,19 @@ func makeAmazonLinux2Config(spec *api.ClusterConfig, ng *api.NodeGroup) (configF
 func NewUserDataForAmazonLinux2(spec *api.ClusterConfig, ng *api.NodeGroup) (string, error) {
 	config := cloudconfig.New()
 
-	scripts := []string{
-		"bootstrap.al2.sh",
-	}
-
 	files, err := makeAmazonLinux2Config(spec, ng)
 	if err != nil {
 		return "", err
 	}
 
+	scripts := []string{}
+
+	if ng.OverrideBootstrapCommand != nil {
+		config.AddShellCommand(*ng.OverrideBootstrapCommand)
+	} else {
+		scripts = append(scripts, "bootstrap.al2.sh")
+	}
+
 	if err = addFilesAndScripts(config, files, scripts); err != nil {
 		return "", err
 	}

pkg/nodebootstrap/userdata_ubuntu.go

@@ -36,15 +36,19 @@ func makeUbuntu1804Config(spec *api.ClusterConfig, ng *api.NodeGroup) (configFil
 func NewUserDataForUbuntu1804(spec *api.ClusterConfig, ng *api.NodeGroup) (string, error) {
 	config := cloudconfig.New()
 
-	scripts := []string{
-		"bootstrap.ubuntu.sh",
-	}
-
 	files, err := makeUbuntu1804Config(spec, ng)
 	if err != nil {
 		return "", err
 	}
 
+	scripts := []string{}
+
+	if ng.OverrideBootstrapCommand != nil {
+		config.AddShellCommand(*ng.OverrideBootstrapCommand)
+	} else {
+		scripts = append(scripts, "bootstrap.ubuntu.sh")
+	}
+
 	if err = addFilesAndScripts(config, files, scripts); err != nil {
 		return "", err
 	}