Made compatible with Python 2.6, required for e.g. RHEL/CentOS 6

Since subprocess.check_output is not available on Python 2.6,
we detect this, and provide an own implementation (lifted from
the official Python sources).
Added option to select ssh cipher; hard-coded selection of arcfour removed

The arcfour cipher has dubious security, and on some systems,
e.g. any which implement CISecurity recommendations, is disabled
in ssh_config.

For the previous behaviour, simply use zreplicate --ssh-cipher=arcfour