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/cninetwork/weave_mac.go

@@ -0,0 +1,10 @@
+// +build darwin
+
+package cninetwork
+
+import "github.com/vishvananda/netlink"
+
+func linkToNetDev(link netlink.Link) (Dev, error) {
+
+	return Dev{}, nil
+}