Hash support for ssh_config

If `['openssh']['client']` is a hash, the template generates a `Host`
for each of the keys and uses the associated values to populate the
host-specific settings.

* templates/default/ssh_config.erb:
Corrected logic for settings

Now also works with string values.

* templates/default/ssh_config.erb:
Updated the README

Added description for the `Hash` configuration option.

* README.md: