make sure CentOS installs wget before installing anything

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
remove undefined arg in pkg_managers

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
Merge pull request #123 from alfredodeza/6482

Ensure `wget` is installed for CentOS