Change mender-artifact and Artifact format v3 from "update" to "payload"

See parent Epic, we defined the taxonomy for the installables inside an Artifact to "payload" (currently a single rootfs image, but will be extended). An update module will be given a single payload to install.

We should change the references in the Artifact format v3 now since it is not yet released. To avoid confusion, user facing parts of mender-artifact should be changed as well.

Acceptance criteria

• Any references as described above are called "payload" in the Mender Artifact format v3, both in relevant file structure and specification
• Any references as described above are called "payload" in the mender-artifact tool options
• mender-artifact write rootfs-image -update argument must be changed to -file.