We had issue to build offline installer on RHEL because of repository difference. This fix enables to build offline installer both on CentOS and RHEL. Also it introduces --releasever <ver>, to build offline installer for specific minor version of CentOS and RHEL. Signed-off-by: Takuya ASADA <syuu@scylladb.com> Message-Id: <20181212032129.29515-1-syuu@scylladb.com>