should support for vagrant plugin config settings

[hoatle]

there are cases that we need to add additional vagrant config, usually when use more custom plugins.

We should not edit Vagrantfile directly but use vagrant_config.json instead (of course, override-able by vagrant_config_override.json, too)

plugins config section like:

{
  "plugins": {
    "vagrant-gatling-rsync": {
      "config_key": "gatling",
      "required": true,
      // auto rsync watch when up/ reload done
      // should try to reduce rsync latency to 0.5s instead of 1s #173
      "latency": 0.5, //0.5s
      "rsync_on_startup": true
    },
    "vagrant-rsync-back": {
      "required": true
    }
  }
}

[hoatle]

this is not easy as I haven't figured out how to set plugin config dynamically.

The current WIP branch: https://github.com/hoatle/teracy-dev/tree/features/186-plugins-settings

[hoatle]

I've merged as incomplete implementation:

• use plugins config format

but temporarily use fixed plugin checking. Need to work on this to get it dynamic.

[hoatle]

related to #239

[hoatle]

new plugin config format from #239

  "plugins": [{
    "_id": "0",
    "name": "vagrant-gatling-rsync",
    "config_key": "gatling",
    "required": true,
    // auto rsync watch when up/ reload done
    // should try to reduce rsync latency to 0.5s instead of 1s #173
    "latency": 0.5, //0.5s
    "time_format": "%H:%M:%S",
    "rsync_on_startup": true
  }, {
    "_id": "1",
    "name": "vagrant-rsync-back",
    "required": true
  }]

[hoatle]

#246 should help to customize other vagrant plugins now, though, it's not the most convenient way (just json config is enough)