storage_local: Use File.open instead of Kernel.open

* https://www.rubydoc.info/gems/rubocop/RuboCop/Cop/Security/Open

```
`Kernel#open` and `URI.open` enable not only file access but also process invocation
by prefixing a pipe symbol (e.g., `open(“| ls”)`).
So, it may lead to a serious security risk by using variable input
to the argument of `Kernel#open` and `URI.open`.
It would be better to use `File.open`, `IO.popen` or `URI.parse#open` explicitly.
```

Signed-off-by: Hiroshi Hatake <hatake@clear-code.com>

lib/fluent/plugin/storage_local.rb

@@ -87,7 +87,7 @@ def configure(conf)
           if File.exist?(@path)
             raise Fluent::ConfigError, "Plugin storage path '#{@path}' is not readable/writable" unless File.readable?(@path) && File.writable?(@path)
             begin
-              data = open(@path, 'r:utf-8') { |io| io.read }
+              data = File.open(@path, 'r:utf-8') { |io| io.read }
               if data.empty?
                 log.warn "detect empty plugin storage file during startup. Ignored: #{@path}"
                 return
@@ -115,7 +115,7 @@ def load
         return if @on_memory
         return unless File.exist?(@path)
         begin
-          json_string = open(@path, 'r:utf-8'){ |io| io.read }
+          json_string = File.open(@path, 'r:utf-8'){ |io| io.read }
           json = Yajl::Parser.parse(json_string)
           unless json.is_a?(Hash)
             log.error "broken content for plugin storage (Hash required: ignored)", type: json.class
@@ -133,7 +133,7 @@ def save
         tmp_path = @path + '.tmp'
         begin
           json_string = Yajl::Encoder.encode(@store, pretty: @pretty_print)
-          open(tmp_path, 'w:utf-8', @mode) { |io| io.write json_string; io.fsync }
+          File.open(tmp_path, 'w:utf-8', @mode) { |io| io.write json_string; io.fsync }
           File.rename(tmp_path, @path)
         rescue => e
           log.error "failed to save data for plugin storage to file", path: @path, tmp: tmp_path, error: e