Adding Filesystem check function to templater

The following code uses the documentFs interface and adds
simple fileExists function to templater plugin.

Change-Id: Ia53c573e54188960eaf99f7c48469359d4e3688b

pkg/document/plugin/templater/templater.go

@@ -27,6 +27,7 @@ import (
 	"sigs.k8s.io/kustomize/kyaml/yaml"
 
 	airshipv1 "opendev.org/airship/airshipctl/pkg/api/v1alpha1"
+	"opendev.org/airship/airshipctl/pkg/fs"
 )
 
 var _ kio.Filter = &plugin{}
@@ -48,11 +49,13 @@ func New(obj map[string]interface{}) (kio.Filter, error) {
 }
 
 func (t *plugin) Filter(items []*yaml.RNode) ([]*yaml.RNode, error) {
+	docfs := fs.NewDocumentFs()
 	out := &bytes.Buffer{}
 	funcMap := sprig.TxtFuncMap()
 	funcMap["toUint32"] = func(i int) uint32 { return uint32(i) }
 	funcMap["toYaml"] = toYaml
 	funcMap["regexGen"] = regexGen
+	funcMap["fileExists"] = docfs.Exists
 	tmpl, err := template.New("tmpl").Funcs(funcMap).Parse(t.Template)
 	if err != nil {
 		return nil, err

pkg/document/plugin/templater/templater_test.go

@@ -181,6 +181,20 @@ template: |
 				"at <regexGen .regex (.limit | int)>: error calling " +
 				"regexGen: error parsing regexp: missing closing ]: `[a-z`",
 		},
+		{
+			cfg: `
+apiVersion: airshipit.org/v1alpha1
+kind: Templater
+metadata:
+  name: notImportantHere
+template: |
+  FileExists: {{ fileExists "./templater.go" }}
+  NoFileExists: {{ fileExists "./templater1.go" }}
+`,
+			expectedOut: `FileExists: true
+NoFileExists: false
+`,
+		},
 	}
 
 	for _, tc := range testCases {