image gc: don't remove images that currently running pods were made from
image gc: don't remove images that currently running pods were made from

This commit changes the behavior of `rkt image gc` to pass over any
images from which currently running pods were created from, regardless
of how long ago the image was last used.

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