distro: add distro_ver for rhel/centos.
distro: move baseurl definition to add_option_distro_centos().
distro: handle baseurl for latest.
Merge pull request #35 from hansode/baseurl-for-latest

Baseurl for latest
distro: fix riken url.
Merge pull request #36 from hansode/fix-riken-url

distro: fix riken url.
Merge pull request #61 from hansode/master

baseurl for latest