Adding useDefaultKeyPair implementation to EngineSSHDialog

Instead of using internal client object, set the keyPair for sshdialog
internally.
It fixes NPE when calling to useDefaultKeyPair without initialize the
client object.

Change-Id: I02edaee6bc6e804bb36d3ab4dabaa21faa926fa0
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>