Avoids the default ssh_config files from reading the default OpenSSH
configuration files.

When specifying an alternative location of OpenSSH configuration,
it is merged into the configurations in the default location since
Net::SSH.start itself loads OpenSSH configurations unless
options[:config] is false.
Follows the previous commit in tests.  Fixes a test failure.
Avoid duplicating the same expectation.
Merge pull request #341 from yugui/feature/alternate-ssh_config

Avoids the default ssh_config files from reading the default OpenSSH configuration files.