api_service: wait until a pod regs with machined
api_service: wait until a pod regs with machined

If we return before we're registered with machined, we could return a
wrong cgroup because the pod is not yet moved to its final cgroup.

Wait until we're registered with machined for 500ms before returning
pod's information.

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