call `yum clean` when installing in RHEL/CentOS

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
Merge pull request #177 from alfredodeza/8034

call `yum clean` when installing in RHEL/CentOS
fix ssh recommendation in docs

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>