flannel: allow input ipam parameters as basis for delegate

This change allows providing an 'ipam' section as part of the
input network configuration for flannel. It is then used as
basis to construct the ipam parameters provided to the delegate.

All parameters from the input ipam are preserved except:
* 'subnet' which is set to the flannel host subnet
* 'routes' which is complemented by a route to the flannel
  network.

One use case of this feature is to allow adding back the routes
to the cluster services and/or to the hosts (HostPort) when
using isDefaultGateway=false. In that case, the bridge plugin
does not install a default route and, as a result, only pod-to-pod
connectivity would be available.

Example:
    {
      "name": "cbr0",
      "cniVersion": "0.3.1",
      "type": "flannel",
      "ipam": {
        "routes": [
          {
            "dst": "192.168.242.0/24"
          },
          {
            "dst": "10.96.0.0/12"
          }
        ],
        "unknown-param": "value"
      },
      "delegate": {
        "hairpinMode": true,
        "isDefaultGateway": false
      }
      ...
    }

This results in the following 'ipam' being provided to the delegate:
    {
      "routes" : [
        {
          "dst": "192.168.242.0/24"
        },
        {
          "dst": "10.96.0.0/12"
        },
        {
          "dst" : "10.1.0.0/16"
        }
      ],
      "subnet" : "10.1.17.0/24",
      "type" : "host-local"
      "unknown-param": "value"
    }

where "10.1.0.0/16" is the flannel network and "10.1.17.0/24" is
the host flannel subnet.

Note that this also allows setting a different ipam 'type' than
"host-local".

Signed-off-by: David Verbeiren <david.verbeiren@tessares.net>

plugins/meta/flannel/flannel.go

@@ -46,6 +46,8 @@ const (
 type NetConf struct {
 	types.NetConf
 
+	// IPAM field "replaces" that of types.NetConf which is incomplete
+	IPAM          map[string]interface{} `json:"ipam,omitempty"`
 	SubnetFile    string                 `json:"subnetFile"`
 	DataDir       string                 `json:"dataDir"`
 	Delegate      map[string]interface{} `json:"delegate"`
@@ -89,6 +91,18 @@ func loadFlannelNetConf(bytes []byte) (*NetConf, error) {
 	return n, nil
 }
 
+func getIPAMRoutes(n *NetConf) ([]types.Route, error) {
+	rtes := []types.Route{}
+
+	if n.IPAM != nil && hasKey(n.IPAM, "routes") {
+		buf, _ := json.Marshal(n.IPAM["routes"])
+		if err := json.Unmarshal(buf, &rtes); err != nil {
+			return rtes, fmt.Errorf("failed to parse ipam.routes: %w", err)
+		}
+	}
+	return rtes, nil
+}
+
 func loadFlannelSubnetEnv(fn string) (*subnetEnv, error) {
 	f, err := os.Open(fn)
 	if err != nil {