Allow Calico networking on Azure and DigitalOcean

* Introduce "calico" as a `networking` option on Azure and DigitalOcean
using Calico's new VXLAN support (similar to flannel). Flannel remains
the default on these platforms for now.
* Historically, DigitalOcean and Azure only allowed Flannel as the
CNI provider, since those platforms don't support IPIP traffic that
was previously required for Calico.
* Looking forward, its desireable for Calico to become the default
across Typhoon clusters, since it provides NetworkPolicy and a
consistent experience
* No changes to AWS, GCP, or bare-metal where Calico remains the
default CNI provider. On these platforms, IPIP mode will always
be used, since its available and more performant than vxlan

azure/container-linux/kubernetes/bootkube.tf

@@ -2,11 +2,18 @@
 module "bootkube" {
   source = "git::https://github.com/poseidon/terraform-render-bootkube.git?ref=85571f6dae3522e2a7de01b7e0a3f7e3a9359641/"
 
-  cluster_name          = "${var.cluster_name}"
-  api_servers           = ["${format("%s.%s", var.cluster_name, var.dns_zone)}"]
-  etcd_servers          = ["${formatlist("%s.%s", azurerm_dns_a_record.etcds.*.name, var.dns_zone)}"]
-  asset_dir             = "${var.asset_dir}"
-  networking            = "flannel"
+  cluster_name = "${var.cluster_name}"
+  api_servers  = ["${format("%s.%s", var.cluster_name, var.dns_zone)}"]
+  etcd_servers = ["${formatlist("%s.%s", azurerm_dns_a_record.etcds.*.name, var.dns_zone)}"]
+  asset_dir    = "${var.asset_dir}"
+
+  networking = "${var.networking}"
+
+  # only effective with Calico networking
+  # we should be able to use 1450 MTU, but in practice, 1410 was needed
+  network_encapsulation = "vxlan"
+  network_mtu           = "1410"
+
   pod_cidr              = "${var.pod_cidr}"
   service_cidr          = "${var.service_cidr}"
   cluster_domain_suffix = "${var.cluster_domain_suffix}"

azure/container-linux/kubernetes/variables.tf

@@ -88,6 +88,12 @@ variable "asset_dir" {
   type        = "string"
 }
 
+variable "networking" {
+  description = "Choice of networking provider (flannel or calico)"
+  type        = "string"
+  default     = "flannel"
+}
+
 variable "host_cidr" {
   description = "CIDR IPv4 range to assign to instances"
   type        = "string"