namefetcher: Use ETag in fetchVerifiedURL()
namefetcher: Use ETag in fetchVerifiedURL()

Makes fetchVerifiedURL() in namefetcher ETag aware to prevent needlessly
re-fetching images.

Fixes #3355

lucasallan/rkt,rkt/rkt,shobhit85/rkt,ybubnov/rkt,intelsdi-x/rkt,kinvolk/rkt,rkt/rkt,Shashwatsh/rkt,coreos/rkt,sstabellini/rkt,joshix/rkt,jakon89/rkt,joshix/rkt,dgonyeo/rkt,sstabellini/rkt,achanda/rkt,jonboulle/rkt,rkt/rkt,alban/rkt,shobhit85/rkt,kinvolk/rkt,cab105/rkt,intelsdi-x/rkt,lucasallan/rkt,dgonyeo/rkt,rkt/rkt,lucasallan/rkt,coreos/rkt,Shashwatsh/rkt,kinvolk/rkt,cab105/rkt,ybubnov/rkt,joshix/rkt,intelsdi-x/rkt,sstabellini/rkt,joshix/rkt,achanda/rkt,jonboulle/rkt,jonboulle/rkt,kinvolk/rkt,lucasallan/rkt,Shashwatsh/rkt,dgonyeo/rkt,ybubnov/rkt,alban/rkt,jakon89/rkt,cab105/rkt,achanda/rkt,shobhit85/rkt,coreos/rkt,alban/rkt,jakon89/rkt