ssh_conn.py: close ssh channel

A simple call to close() resulted in crash with Python Error, starting from libssh2.6. The current workaround is a try-except around the close() and another call to wait_closed() in the except block.

See GitHub Issue #49.

Signed-off-by: Peter Winkler <winkler@dresearch-fe.de>
Acked-by: Erik Bernoth <bernoth@dresearch-fe.de>
Acked-by: Steffen Sledz <sledz@dresearch-fe.de>
test_ssh_conn: added unit test for close handling

See GitHub Issue #49.

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>
Acked-by: Peter Winkler <winkler@dresearch-fe.de>
Acked-by: Steffen Sledz <sledz@dresearch-fe.de>