SSH: Set keep-alive timer period to connection timeout.

The Qt SDK's Qemu can be so slow that a single packet takes more
than the old fixed value of five seconds for a round trip.

Change-Id: I06f442c3c080fec8d1b1d10bd26e2e27b06e6863
Reviewed-on: http://codereview.qt.nokia.com/854


Reviewed-by: Christian Kandeler <christian.kandeler@nokia.com>