ipatests: coerce tmpdir to string
    
    In testcase test_ssh_key_connection the value provided by tmpdir fixture
    is used as regular string, i.e. passed as an argument to os.path.join.
    tmpdir is an instance of py.path.local, and in old versions of package
    python-py it does not have methods of string object,
    which causes this test to fail in distributions where such version of
    python-py is installed.
    Fixed by explicitly changing type of tmpdir to str.
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>