Add depends_on field for core service ordering

* Adds depends_on fields to compose YAML
* Updates parsing code to copy across depends_on field to
openfaas service from compose service definition
* Adds algorithm and unit tests for finding order
* Applies order to up.go command
* Makes unit testing on MacOS possible through build directives

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

pkg/depends_on.go

@@ -0,0 +1,35 @@
+package pkg
+
+import "fmt"
+
+func buildInstallOrder(svcs []Service) []string {
+	graph := Graph{nodes: []*Node{}}
+
+	nodeMap := map[string]*Node{}
+	for _, s := range svcs {
+		n := &Node{Name: s.Name}
+		nodeMap[s.Name] = n
+		graph.nodes = append(graph.nodes, n)
+	}
+
+	for _, s := range svcs {
+		for _, d := range s.DependsOn {
+			nodeMap[s.Name].Edges = append(nodeMap[s.Name].Edges, nodeMap[d])
+		}
+	}
+
+	resolved := &Graph{}
+	unresolved := &Graph{}
+	for _, g := range graph.nodes {
+		resolve(g, resolved, unresolved)
+	}
+
+	fmt.Printf("Installation order:\n")
+	order := []string{}
+	for _, node := range resolved.nodes {
+		fmt.Printf("- %s\n", node.Name)
+		order = append(order, node.Name)
+	}
+
+	return order
+}