Modify prebuilt images

There are several use-cases around the need for modifying .sdimg and .mender files after they have been built. Internally we need to customize the device run-time for each customer as part of the SaaS offering. Externally we need to make it easier to get started with Mender's prebuilt images - right now there are too many steps required to set up the networking and keys in .sdimg and .mender files.

Acceptance criteria

• The tenant token of QEMU images can be injected during run-time (so we don't have to run different images for each customer on the SaaS platform)
• mender-artifact makes it easy to modify a .mender Artifact's 1) Mender Server URI, 2) server.crt 3) tenant token and 4) signature verification key
• mender-artifact makes it easy to modify a .sdmig's 1) Mender Server URI, 2) server.crt 3) tenant token and 4) signature verification key
• when using mender-artifact -n the name of the artifact is also updated inside the rootfs (see current modify docs and the -n caveat with /etc/mender/artifact_info which should be automatically handled)
• if Artifact was signed when using modify option, mender-artifact would generate an error and ask for signing key.
• (stretch goal) it is easy to modify any file in the file systems in either images (Artifact rootfs & disk image)