feat(templater): override sprig default function with guarded alternative

These changes overrides the `default` function provided by sprig with
an alternative to retrieve variable values from variables that might
not have been declared at all.

Referencing a variable in a template that is not declared, will lead
to the underlying templating functionality raising an error, causing
kontemplate to exit.

The override alternative to `default` accepts a second string argument
with the variable name. If the variable in question has not been
declared the first argument's value would be returned, just as the
original `default` function does.

templater/templater.go

@@ -203,7 +203,13 @@ func templateFuncs(c *context.Context, rs *context.ResourceSet) template.FuncMap
 
 		return string(data), nil
 	}
+	m["default"] = func(defaultVal interface{}, varName string) interface{} {
+		if val, ok := rs.Values[varName]; ok {
+			return val
+		}
 
+		return defaultVal
+	}
 	return m
 }