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/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"