upgrade terraform to V0.12.12 (#22)

* upgrade terraform to V0.12.12

* fix seed value for gcp example

examples/gcp/main.tf

@@ -1,5 +1,5 @@
 provider "gardener" {
-  kube_file          = "${file("<my-gardener-service-account-kubeconfig>")}"
+  kube_file = "${file("<my-gardener-service-account-kubeconfig>")}"
   /*kube_file          =<<-EOT
     kind: Config
     clusters:
@@ -35,7 +35,7 @@ resource "gardener_shoot" "test_cluster" {
     cloud {
       profile = "gcp"
       region  = "europe-west3"
-      seed    = "azure-eu2"
+      seed    = "gcp-eu1"
       secret_binding_ref {
         name = "<secret_binding>"
       }

go.mod

@@ -3,26 +3,19 @@ module github.com/kyma-incubator/terraform-provider-gardener
 go 1.13
 
 require (
-	github.com/aws/aws-sdk-go v1.25.2 // indirect
 	github.com/gardener/gardener v0.0.0-20190906111529-f9ad04069615
 	github.com/gogo/protobuf v1.3.0 // indirect
-	github.com/google/go-cmp v0.3.1 // indirect
-	github.com/googleapis/gax-go v2.0.0+incompatible // indirect
-	github.com/hashicorp/go-cleanhttp v0.5.1 // indirect
 	github.com/hashicorp/go-hclog v0.9.2 // indirect
 	github.com/hashicorp/go-plugin v1.0.1 // indirect
-	github.com/hashicorp/go-version v1.2.0 // indirect
 	github.com/hashicorp/hcl2 v0.0.0-20190909202536-66c59f909e25 // indirect
 	github.com/hashicorp/hil v0.0.0-20190212132231-97b3a9cdfa93 // indirect
-	github.com/hashicorp/terraform v0.11.14
+	github.com/hashicorp/terraform v0.12.12
 	github.com/hashicorp/yamux v0.0.0-20190923154419-df201c70410d // indirect
 	github.com/mattn/go-isatty v0.0.9 // indirect
 	github.com/mitchellh/reflectwalk v1.0.1 // indirect
 	github.com/spf13/pflag v1.0.5 // indirect
 	github.com/ugorji/go/codec v1.1.7 // indirect
-	github.com/zclconf/go-cty v1.1.0 // indirect
 	golang.org/x/crypto v0.0.0-20191001141032-4663e185863a // indirect
-	golang.org/x/net v0.0.0-20190930134127-c5a3c61f89f3 // indirect
 	golang.org/x/sys v0.0.0-20190927073244-c990c680b611 // indirect
 	golang.org/x/time v0.0.0-20190921001708-c4c64cad1fd0 // indirect
 	google.golang.org/appengine v1.6.4 // indirect