test_integration: Add OpenSSHTransport, used if paramiko is not available
    
    This adds a transport that uses /usr/bin/ssh calls to communicate
    with remote hosts.
    This transport is a bit slower and buffers output more than paramiko,
    so it is only used if paramiko is not available, or forced with an
    environment variable.
    
    https://fedorahosted.org/freeipa/ticket/3890