distro: add distro/centos7.
Merge pull request #32 from hansode/centos7

distro: add distro/centos7.
distro: generate /etc/default/grub in install_menu_lst_grub2.
Merge pull request #33 from hansode/fix-grub2-config

distro: generate /etc/default/grub in install_menu_lst_grub2.
distro: use vault.centos.org instead of ftp.riken.co.jp.
Merge pull request #34 from hansode/use-vault

distro: use vault.centos.org instead of ftp.riken.co.jp.
Merge pull request #60 from hansode/master

use vault instead of riken