should support local vagrant plugins

[hoatle]

local plugins should be within the plugins directory for prod usage. With dev mode, a plugin can be placed within the workspace directory, follow the convention of teracy-dev extensions for location lookup and path.

When a plugin is updated, make sure to re-install the plugin (gem) so that vagrant gets the latest code of the plugin.

I guess we could introduce: is_local and location option for plugin config so that we know it's a local plugin to handle properly.

[hoatle]

I guess we don't need this yet for v0.6, so maybe we could make this into v0.7 or when we really need this feature

[hoatle]

related: #493

As I could see, plugin_manager also supports local plugin so we just need to add config support

[hoatle]

vagrant introduced local and global mode plugins, I need to specify more to avoid confusions.