untangle.py: use a configurable "timeout" for socket operations.

If an untangle host holds a connection open but sends no data, it can
hang the backup process because we'll simply wait forever on that one
host. To work around that problem, a new configurable "timeout" value
has been added and is used in all open() calls. When the timeout is
reached, a socket.timeout error is raised, terminating the backup
process for that host (and all others, at the moment).