Rename `base::ntpdate` to `base::ntp` and install `ntp` package.

This commit refs #67.
Update `/etc/ntp.conf`.

This commit refs #67.
Restart `ntp` service, if the configuration file is updated.

This commit refs #67.
Add tests for installing `ntp`.

This commit refs #67.
Merge pull request #67 from kazu634/id/67

ntpdateではなく、ntp daemon を利用し時刻同期する