Override default variables with SystemConfig

[cosmo0920]

I've implemented to override default variables with system config mechanism.

This PR is related to this developer task: make process global constants obsoleve in fluent/env

ref: https://github.com/fluent/fluentd/wiki/Developer-notes-for-fixes-on-v0.14#plugin-miscellaneous

Remaining Task(s)

• Write test code to confirm override feature
  • test code for overriding port
  • test code for overriding file permission
  • test code for overriding dir permission
    • out_file
    • buf_file
  • Omit permission related tests when running on Windows
  • remove DEFAULT_* from env.rb & add default values of ports or permissions to each plugins
  • remove xxx_override methods and use perm = @perm_param_of_plugin || system_config.file_permissions || PLUGIN_DEFAULT_PERM in plugin
  • squash puzzling commits
  • Tidying up tests
  • Remove port number overriding mechanism. Because it is useless.

Question(s)

• Where is suitable place to put into obsoleted variables? Or, leave them as-is is enough? Solved.

[cosmo0920]

Oh, NTFS does not support UNIX like permissions!
It should do more work for Windows.... :<

[tagomoris]

system_config_override is to override instance variables for each plugin instances. Once it was overridden, following system_config method refers instance variable, not global system config instance.
(In other words, plugin just refer system_config even in tests after system_config_override is called in setup or somewhere else.)

So, there's no need to add xxx_override methods.

[tagomoris]

Default values of port, permission and others are for each plugins, not for Fluentd global.
The best way to fix it is:

• remove DEFAULT_* from env.rb
• add default values of ports or permissions to each plugins

Especially for file/directory permissions, it's better to provide system config parameters because there's not so variations about it (in many cases, permission control policy is system-wide).

  perm = @perm_param_of_plugin || system_config.file_permissions || PLUGIN_DEFAULT_PERM

[tagomoris]

Could you remove this line?
There's no need to configure system-wide listen_port because this configuration cannot be shared by anything.
Removing this line also makes port_override methods useless.

[cosmo0920]

OK. I'll do it.

[repeatedly]

@tagomoris How about this? Can we merge this?

[tagomoris]

LGTM. I'll merge this right now.

[cosmo0920]

Thanks!