stage0/app-add: CLI args should override image ones
stage0/app-add: CLI args should override image ones

This commit uniforms app args and exec command, so that in both cases
CLI parameters always override image ones.

dgonyeo/rkt,joshix/rkt,shobhit85/rkt,rkt/rkt,achanda/rkt,joshix/rkt,alban/rkt,lucasallan/rkt,jonboulle/rkt,coreos/rkt,cab105/rkt,Shashwatsh/rkt,joshix/rkt,rkt/rkt,cab105/rkt,rkt/rkt,coreos/rkt,dgonyeo/rkt,alban/rkt,shobhit85/rkt,ybubnov/rkt,sstabellini/rkt,kinvolk/rkt,intelsdi-x/rkt,lucasallan/rkt,alban/rkt,ybubnov/rkt,Shashwatsh/rkt,achanda/rkt,achanda/rkt,rkt/rkt,intelsdi-x/rkt,joshix/rkt,kinvolk/rkt,ybubnov/rkt,kinvolk/rkt,shobhit85/rkt,jonboulle/rkt,lucasallan/rkt,Shashwatsh/rkt,kinvolk/rkt,jonboulle/rkt,cab105/rkt,intelsdi-x/rkt,lucasallan/rkt,dgonyeo/rkt,sstabellini/rkt,coreos/rkt,sstabellini/rkt