scripts: introduce a new helper script glide-update.sh
scripts: introduce a new helper script glide-update.sh

Introduce a new helper script glide-update.sh to be able to easily
update glide vendors.
This script was originally taken from rkt.

giantswarm/fleet,giantswarm/fleet,coreos/fleet,dalbani/fleet,coreos/fleet,dalbani/fleet,jonboulle/fleet,dongsupark/fleet,endocode/fleet,endocode/fleet,dongsupark/fleet,giantswarm/fleet,endocode/fleet,jonboulle/fleet,coreos/fleet,dalbani/fleet,jonboulle/fleet,dongsupark/fleet