fix(ssh): Only set HostKeyCallback if not nil
fix(ssh): Only set HostKeyCallback if not nil

Fix #248

wuqixuan/fleet,bviswana101/fleet,ecnahc515/fleet,dongsupark/fleet,mwitkow/fleet,esatterwhite/fleet,endocode/fleet,giantswarm/fleet,dalbani/fleet,wuqixuan/fleet,coreos/fleet,webwlsong/fleet,WPMedia/fleet,mischief/fleet,mwitkow-io/fleet,dongsupark/fleet,dalbani/fleet,jonboulle/fleet,jonboulle/fleet,dalbani/fleet,flandr/fleet,webwlsong/fleet,mookjp/fleet,thepwagner/fleet,bcwaldon/fleet,endocode/fleet,coreos/fleet,bcwaldon/fleet,paveltiunov/fleet,mookjp/fleet,bcwaldon/fleet,mwitkow/fleet,octoblu/fleet,esatterwhite/fleet,paveltiunov/fleet,improbable-io/fleet,coreos/fleet,bviswana101/fleet,mwitkow-io/fleet,hhoover/fleet,WPMedia/fleet,jonboulle/fleet,mischief/fleet,improbable-io/fleet,mwhooker/fleet,improbable-io/fleet,giantswarm/fleet,ecnahc515/fleet,dongsupark/fleet,flandr/fleet,giantswarm/fleet,thepwagner/fleet,ecnahc515/fleet,mwhooker/fleet,octoblu/fleet,hhoover/fleet,endocode/fleet