Remove aks-engine from "vendor/" and go.mod. (Azure#277)

* Remove aks-engine from "vendor/" and go.mod.
I have previously removed all dependencies on aks-engine so we don't need it anymore.

* Add modules depended upon by aks-engine to our go.mod.
Previously I removed aks-engine from our go.mod file, now I got this build error:

../../../go/pkg/mod/k8s.io/[email protected]+incompatible/rest/request.go:598:31: not enough arguments in call to watch.NewStreamWatcher
	have (*versioned.Decoder)
	want (watch.Decoder, watch.Reporter)

It seems aks-engine depends on some older versions of some modules. After removing aks-engine from our go.mod, our code now would try to get newer, incompatible versions of the modules and thus the build break. Adding the depended upon module versions explicitly to the go.mod file fixes the problem. The versions are obtained from aks-engine's go.mod file (https://github.com/Azure/aks-engine/blob/v0.47.0-aks-gomod-a5/go.mod)

go.mod

@@ -3,9 +3,9 @@ module github.com/Azure/agentbaker
 go 1.12
 
 require (
-	github.com/Azure/aks-engine v0.47.0-aks-gomod-a5
 	github.com/Azure/go-autorest/autorest v0.9.2
 	github.com/Azure/go-autorest/autorest/to v0.3.0
+	github.com/Jeffail/gabs v1.1.1
 	github.com/blang/semver v3.5.1+incompatible
 	github.com/go-playground/universal-translator v0.17.0 // indirect
 	github.com/google/go-cmp v0.4.0
@@ -14,6 +14,7 @@ require (
 	github.com/leodido/go-urn v1.2.0 // indirect
 	github.com/leonelquinteros/gotext v1.4.0
 	github.com/mattn/go-colorable v0.0.9
+	github.com/mattn/go-isatty v0.0.10 // indirect
 	github.com/onsi/ginkgo v1.12.2
 	github.com/onsi/gomega v1.10.1
 	github.com/pkg/errors v0.9.1
@@ -23,5 +24,7 @@ require (
 	golang.org/x/sys v0.0.0-20200523222454-059865788121 // indirect
 	gopkg.in/go-playground/validator.v9 v9.31.0 // indirect
 	gopkg.in/ini.v1 v1.57.0
+	k8s.io/apimachinery v0.0.0-20190221213512-86fb29eff628
 	k8s.io/client-go v10.0.0+incompatible
+	k8s.io/klog v1.0.0 // indirect
 )