Force connection auth as part of RemoteClient in scenario

Some operations use ssh.exec_command() (or maybe other methods) without
first verifying that ssh connection is available. This might cause ambiguity
when debugging failed tests.
This patch forces connection verification as part of ssh_client creation in
scenario tests, and will cause early failure if connection is faulty

Change-Id: I002fee2a231e48e834f80f62cc70a489eaa8d2c1
Merge "Force connection auth as part of RemoteClient in scenario"