plugins/ipam: round robin ip allocation for host-local ipam

This changes the ip allocation logic to round robin. Before this, host-local IPAM searched for available IPs from start of subnet. Hence it tends to allocate IPs that had been used recently. This is not ideal since it may cause collisions.

plugins/ipam/host-local/config.go

@@ -60,7 +60,7 @@ func LoadIPAMConfig(bytes []byte, args string) (*IPAMConfig, error) {
 	}
 
 	if n.IPAM == nil {
-		return nil, fmt.Errorf("%q missing 'ipam' key")
+		return nil, fmt.Errorf("IPAM config missing 'ipam' key")
 	}
 
 	// Copy net name into IPAM so not to drag Net struct around