fleetd: support the version command
fleetd: support the version command

Now support "fleetd version" and "fleetd --version"

Fixes #1210

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