Honor CNI and bridge related fields

Nomad agent may silently ignore cni_path and bridge setting, when it
merges configs from multiple files (or against default/dev config).

This PR ensures that the values are merged properly.

command/agent/config.go

@@ -270,9 +270,6 @@ type ClientConfig struct {
 	// available to jobs running on this node.
 	HostVolumes []*structs.ClientHostVolumeConfig `hcl:"host_volume"`
 
-	// ExtraKeysHCL is used by hcl to surface unexpected keys
-	ExtraKeysHCL []string `hcl:",unusedKeys" json:"-"`
-
 	// CNIPath is the path to search for CNI plugins, multiple paths can be
 	// specified colon delimited
 	CNIPath string `hcl:"cni_path"`
@@ -285,6 +282,9 @@ type ClientConfig struct {
 	// creating allocations with bridge networking mode. This range is local to
 	// the host
 	BridgeNetworkSubnet string `hcl:"bridge_network_subnet"`
+
+	// ExtraKeysHCL is used by hcl to surface unexpected keys
+	ExtraKeysHCL []string `hcl:",unusedKeys" json:"-"`
 }
 
 // ClientTemplateConfig is configuration on the client specific to template
@@ -1480,6 +1480,16 @@ func (a *ClientConfig) Merge(b *ClientConfig) *ClientConfig {
 		result.HostVolumes = structs.HostVolumeSliceMerge(a.HostVolumes, b.HostVolumes)
 	}
 
+	if b.CNIPath != "" {
+		result.CNIPath = b.CNIPath
+	}
+	if b.BridgeNetworkName != "" {
+		result.BridgeNetworkName = b.BridgeNetworkName
+	}
+	if b.BridgeNetworkSubnet != "" {
+		result.BridgeNetworkSubnet = b.BridgeNetworkSubnet
+	}
+
 	return &result
 }