Strengthen the permission for the generated ssh key
Strengthen the permission for the generated ssh key

As the ssh server requires private keys to have strict permissions, provides
them so that they are immediately usable.

Also change the opening mode to a more appropriate one.

Change-Id: I330c0aadf40b364c46801aa916493c367de25a8b

openstack/sahara-tests,openstack/sahara-scenario,openstack/sahara-tests,openstack/sahara-tests,openstack/sahara-scenario,openstack/sahara-tests,openstack/sahara-scenario