feat(interfaces): Add support for interfaces file

This adds the ability for cloudinit to parse a debian interfaces
file and generate the coresponding networkd configs.

network/network.go

@@ -0,0 +1,86 @@
+package network
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"path"
+	"strings"
+)
+
+func ProcessDebianNetconf(config string) ([]InterfaceGenerator, error) {
+	lines := formatConfig(config)
+	stanzas, err := parseStanzas(lines)
+	if err != nil {
+		return nil, err
+	}
+
+	interfaces := make([]*stanzaInterface, 0, len(stanzas))
+	for _, stanza := range stanzas {
+		switch s := stanza.(type) {
+		case *stanzaInterface:
+			interfaces = append(interfaces, s)
+		}
+	}
+
+	return buildInterfaces(interfaces), nil
+}
+
+func WriteConfigs(configPath string, interfaces []InterfaceGenerator) error {
+	if err := os.MkdirAll(configPath, os.ModePerm+os.ModeDir); err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+
+	for _, iface := range interfaces {
+		filename := path.Join(configPath, fmt.Sprintf("%s.netdev", iface.Name()))
+		if err := writeConfig(filename, iface.GenerateNetdevConfig()); err != nil {
+			return err
+		}
+		filename = path.Join(configPath, fmt.Sprintf("%s.link", iface.Name()))
+		if err := writeConfig(filename, iface.GenerateLinkConfig()); err != nil {
+			return err
+		}
+		filename = path.Join(configPath, fmt.Sprintf("%s.network", iface.Name()))
+		if err := writeConfig(filename, iface.GenerateNetworkConfig()); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func writeConfig(filename string, config string) error {
+	if config == "" {
+		return nil
+	}
+
+	if file, err := os.Create(filename); err == nil {
+		io.WriteString(file, config)
+		file.Close()
+		return nil
+	} else {
+		return err
+	}
+}
+
+func formatConfig(config string) []string {
+	lines := []string{}
+	config = strings.Replace(config, "\\\n", "", -1)
+	for config != "" {
+		split := strings.SplitN(config, "\n", 2)
+		line := strings.TrimSpace(split[0])
+
+		if len(split) == 2 {
+			config = split[1]
+		} else {
+			config = ""
+		}
+
+		if strings.HasPrefix(line, "#") || line == "" {
+			continue
+		}
+
+		lines = append(lines, line)
+	}
+	return lines
+}