Fixes the following vet reports:
builder/openstack/artifact.go:44: arg a.ImageId for printf verb %d of wrong type: string
builder/openstack/step_wait_for_rackconnect.go:24: arg server for printf verb %s of wrong type: *github.com/mitchellh/gophercloud-fork-40444fb.Server
In order that something consuming an artifact can have access to extra
builder specific data add the State method which allows the caller to
ask for arbitary values by name.
Mitchell Hashimoto
Emil Hessman
Mitchell Hashimoto
Julian Phillips
Mark Peek