connectors: refactor filter code into a helper package

I hope I didn't miss any :D

Signed-off-by: Stephan Renatus <srenatus@chef.io>

pkg/groups/groups.go

@@ -0,0 +1,18 @@
+// Package groups contains helper functions related to groups
+package groups
+
+// Filter filters out any groups of given that are not in required. Thus it may
+// happen that the resulting slice is empty.
+func Filter(given, required []string) []string {
+	groups := []string{}
+	groupFilter := make(map[string]struct{})
+	for _, group := range required {
+		groupFilter[group] = struct{}{}
+	}
+	for _, group := range given {
+		if _, ok := groupFilter[group]; ok {
+			groups = append(groups, group)
+		}
+	}
+	return groups
+}