Add pkg/discovery for nodes discovery

Absorb Swarm's discovery package in order to provide a common node
discovery mechanism to be used by both Swarm and networking code.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>

discovery/generator.go

@@ -0,0 +1,35 @@
+package discovery
+
+import (
+	"fmt"
+	"regexp"
+	"strconv"
+)
+
+// Generate takes care of IP generation
+func Generate(pattern string) []string {
+	re, _ := regexp.Compile(`\[(.+):(.+)\]`)
+	submatch := re.FindStringSubmatch(pattern)
+	if submatch == nil {
+		return []string{pattern}
+	}
+
+	from, err := strconv.Atoi(submatch[1])
+	if err != nil {
+		return []string{pattern}
+	}
+	to, err := strconv.Atoi(submatch[2])
+	if err != nil {
+		return []string{pattern}
+	}
+
+	template := re.ReplaceAllString(pattern, "%d")
+
+	var result []string
+	for val := from; val <= to; val++ {
+		entry := fmt.Sprintf(template, val)
+		result = append(result, entry)
+	}
+
+	return result
+}