Rename the test target node name to `sandbox`.

This commit refs #33.
Rename the test target node name to `sandbox-test`.

This commit refs #33.
Modify the SSH configuration file.

This commit refs #33.
Delete the `toggle_ssh_config.rake`.

This commit refs #33.
Install gems, by using `rbenv-default-gems`.

In order to workaround the problem of failing Ruby installation,
use `rbenv-default-gems` plugin to install the gems
(Ruby instllation fails, maybe because of chef's `gems` command specification):

- no longer supports `gems` line of `ruby_setup` LWRP.
- instead, installs `rbenv-default-gems` plugin.
- specify the gems to install, by `default-gems`.

This commit refs #34.
Merge pull request #34 from kazu634/id/34

Rubyインストール時のgemインストールで失敗する