build: shorten the default stage1-image
build: shorten the default stage1-image

The long stage1-image name was making the help output for options in the
run and prepare commands look bad, This is a cosmetic change that makes
the option descriptions less likely to have to wrap.

jjlakis/rkt,lucasallan/rkt,adfernandes/rkt,kinvolk/rkt,blixtra/rkt,yifan-gu/rocket,sgotti/rkt,squeed/rkt,jasonchaffee/rkt,shobhit85/rkt,coreos/rkt,joshix/rkt,dgonyeo/rkt,adfernandes/rkt,Shashwatsh/rkt,linki/rkt,mike1729/rkt,matthaias/rkt,mike1729/rkt,linki/rkt,il9ue/rkt,s-urbaniak/rkt,matthaias/rkt,jakon89/rkt,il9ue/rkt,yifan-gu/rocket,tmrts/rkt,adfernandes/rkt,squeed/rkt,jonboulle/rkt,kbrwn/rkt,tmrts/rkt,cab105/rkt,rkt/rkt,kbrwn/rkt,lucasallan/rkt,lucasallan/rkt,kbrwn/rkt,krnowak/rkt,cab105/rkt,euank/rkt,shobhit85/rkt,markllama/rkt,akshaykarle/rkt,linki/rkt,dgonyeo/rkt,il9ue/rkt,jonboulle/rkt,Wilybald/rkt,sinfomicien/rkt,coreos/rkt,endocode/rkt,rkt/rkt,nhlfr/rkt,jakon89/rkt,markllama/rkt,s-urbaniak/rkt,rkt/rkt,blixtra/rkt,jakon89/rkt,sstabellini/rkt,euank/rkt,liugenping/rkt,dgonyeo/rkt,yifan-gu/rkt,kinvolk/rkt,coreos/rkt,shobhit85/rkt,jzelinskie/rkt,monstermunchkin/rkt,Shashwatsh/rkt,sinfomicien/rkt,endocode/rkt,ybubnov/rkt,endocode/rkt,achanda/rkt,sgotti/rkt,cab105/rkt,jonboulle/rkt,alban/rkt,jjlakis/rkt,alban/rkt,blixtra/rkt,yifan-gu/rkt,alban/rkt,intelsdi-x/rkt,monstermunchkin/rkt,joshix/rkt,Shashwatsh/rkt,krnowak/rkt,liugenping/rkt,sinfomicien/rkt,liugenping/rkt,akshaykarle/rkt,Wilybald/rkt,intelsdi-x/rkt,s-urbaniak/rkt,jzelinskie/rkt,joshix/rkt,Wilybald/rkt,achanda/rkt,rkt/rkt,joshix/rkt,nhlfr/rkt,squeed/rkt,jzelinskie/rkt,mike1729/rkt,jjlakis/rkt,jasonchaffee/rkt,ybubnov/rkt,yifan-gu/rkt,Wilybald/rkt,tmrts/rkt,sgotti/rkt,yifan-gu/rocket,intelsdi-x/rkt,matthaias/rkt,jasonchaffee/rkt,lucasallan/rkt,nhlfr/rkt,ybubnov/rkt,akshaykarle/rkt,kinvolk/rkt,monstermunchkin/rkt,sstabellini/rkt,sstabellini/rkt,euank/rkt,krnowak/rkt,achanda/rkt,kinvolk/rkt,markllama/rkt