Update targetOs to target_os to fix packer errors (#46)

The targetOs version of the parameter wasn't usable by packer directly,
and I received an error attempting to use the merged version of the
changes to use the 1.7 packer release. Turns out that when I rebased
onto the windows support, I failed to directly test with the same issue
reported in issue 40
[here](#40 (comment))

I fixed a couple of issues around struct tags when updating the
provisioner to support 1.7; I just happened to miss the new parameter
added with Windows support.

README.md

@@ -50,7 +50,7 @@ There is an example packer build with goss tests in the `example/` directory.
     "format": "",
     "goss_file": "",
     "vars_file": "",
-    "targetOs": "Linux",
+    "target_os": "Linux",
     "vars_env": {
       "ARCH": "amd64",
       "PROVIDER": "{{user `cloud-provider`}}"
@@ -70,7 +70,7 @@ Goss spec file and debug spec file (`goss render -d`) are downloaded to `/tmp` f
 
 ## Windows support
 
-This now has support for Windows. Set the optional parameter `targetOs` to `Windows`. Currently, the `vars_env` parameter must include `GOSS_USE_ALPHA=1` as specified in [goss's feature parity document](https://github.com/aelsabbahy/goss/blob/master/docs/platform-feature-parity.md#platform-feature-parity).  In the future when goss come of of alpha for Windows this parameter will not be required.
+This now has support for Windows. Set the optional parameter `target_os` to `Windows`. Currently, the `vars_env` parameter must include `GOSS_USE_ALPHA=1` as specified in [goss's feature parity document](https://github.com/aelsabbahy/goss/blob/master/docs/platform-feature-parity.md#platform-feature-parity).  In the future when goss come of of alpha for Windows this parameter will not be required.
 
 ## Installation
 

packer-provisioner-goss.go

@@ -37,7 +37,7 @@ type GossConfig struct {
 	Password     string
 	SkipInstall  bool `mapstructure:"skip_install"`
 	Inspect      bool
-	TargetOs     string
+	TargetOs     string `mapstructure:"target_os"`
 
 	// An array of tests to run.
 	Tests []string

packer-provisioner-goss_test.go

@@ -72,8 +72,8 @@ func TestProvisioner_Prepare(t *testing.T) {
 			name: "Windows",
 			input: []interface{}{
 				map[string]interface{}{
-					"tests":    []string{"example/goss"},
-					"targetOs": "Windows",
+					"tests":     []string{"example/goss"},
+					"target_os": "Windows",
 					"vars_env": map[string]string{
 						"GOSS_USE_ALPHA": "1",
 					},
@@ -112,8 +112,8 @@ func TestProvisioner_Prepare(t *testing.T) {
 			name: "Windows non alpha",
 			input: []interface{}{
 				map[string]interface{}{
-					"tests":    []string{"example/goss"},
-					"targetOs": "Windows",
+					"tests":     []string{"example/goss"},
+					"target_os": "Windows",
 				},
 			},
 			wantErr: false,