Bug 1903: Better SSH check

Changes the ssh port ready check from a simple 'is the port open?' to
a full ssh login check to ensure that sshd is actually running on the
target server.

Adds timeout to ssh check

Fixes bug 1903

Change-Id: I8c3f0d8e1ac450726bc3336c00e7fbdf405fe067
Signed-off-by: beierlm <mark.beierl@canonical.com>