rkt: use exit status 2 to report usage errors
rkt: use exit status 2 to report usage errors

An obsolete CoreOS test was erroneously passing:

    Error: unknown command "install" for "rkt"
    Run 'rkt --help' for usage.
    2016-02-08T00:57:26Z kola: --- PASS: coreos.rkt.install

Wilybald/rkt,il9ue/rkt,shobhit85/rkt,intelsdi-x/rkt,jakon89/rkt,s-urbaniak/rkt,yifan-gu/rocket,alban/rkt,jjlakis/rkt,yifan-gu/rkt,sgotti/rkt,mike1729/rkt,yifan-gu/rkt,yifan-gu/rocket,cab105/rkt,nhlfr/rkt,lucasallan/rkt,Wilybald/rkt,joshix/rkt,cab105/rkt,squeed/rkt,joshix/rkt,monstermunchkin/rkt,alban/rkt,il9ue/rkt,akshaykarle/rkt,s-urbaniak/rkt,mike1729/rkt,dgonyeo/rkt,rkt/rkt,jjlakis/rkt,coreos/rkt,yifan-gu/rocket,alban/rkt,ybubnov/rkt,euank/rkt,kinvolk/rkt,coreos/rkt,dgonyeo/rkt,kinvolk/rkt,matthaias/rkt,sstabellini/rkt,tmrts/rkt,achanda/rkt,Shashwatsh/rkt,monstermunchkin/rkt,kinvolk/rkt,coreos/rkt,yifan-gu/rkt,squeed/rkt,intelsdi-x/rkt,sstabellini/rkt,nhlfr/rkt,jonboulle/rkt,shobhit85/rkt,markllama/rkt,nhlfr/rkt,rkt/rkt,sstabellini/rkt,cab105/rkt,jjlakis/rkt,jonboulle/rkt,shobhit85/rkt,joshix/rkt,tmrts/rkt,matthaias/rkt,Shashwatsh/rkt,rkt/rkt,il9ue/rkt,matthaias/rkt,Wilybald/rkt,jakon89/rkt,ybubnov/rkt,squeed/rkt,achanda/rkt,tmrts/rkt,jakon89/rkt,mike1729/rkt,ybubnov/rkt,akshaykarle/rkt,kinvolk/rkt,Wilybald/rkt,dgonyeo/rkt,intelsdi-x/rkt,euank/rkt,rkt/rkt,s-urbaniak/rkt,lucasallan/rkt,achanda/rkt,akshaykarle/rkt,sgotti/rkt,lucasallan/rkt,markllama/rkt,markllama/rkt,sgotti/rkt,joshix/rkt,monstermunchkin/rkt,lucasallan/rkt,Shashwatsh/rkt,jonboulle/rkt,euank/rkt