Fix ssh host key checking with no-key-check

In case we add a node with “--no-ssh-key-check”, this should override
any default yes/ask values in the system-wide (or user) ssh key check.

Currently this only works in batch mode, whereas in non-batch we only
override a 'no'. The patch fixes SshRunner such that in non-batch mode
we enforce the value of StrictHostKeyChecking in all cases.

Bug found and initial investigation by Theo Van Dinter.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>