Replace Net::SSH2 with Net::OpenSSH in dependencies

[ferki]

Net::OpenSSH is the preferred choice internally, let's use that as a dependency for the packages as well.

[ferki]

Looks like at least Ubuntu 16.04 packages for rex-1.4.1 are still specifying Net::SSH2 as their dependency instead of Net::OpenSSH+Net::SFTP::Foreign.