Support uncompressed updates

Currently the header (header.tar.gz) and updates (e.g. data/0000.tar.gz, data/0001.tar.gz) in Mender Artifacts are assumed to be compressed (with .gz). See the Artifact format specification for more details.

However lower-end devices like microcontrollers have a challenge working with compressed data due to constrained resources. See this community list thread for more information: https://groups.google.com/a/lists.mender.io/forum/#!topic/mender/iW53Wc0na0w

For this reason it would be nice if it was possible to support uncompressed tar files as well (e.g. data/0000.tar, data/0001.tar).

Since the mender artifact library (and thus Mender server) computes the checksum of the inside of this tar file (the rootfs image), this requires some changes in the mender-artifact library.

Acceptance criteria

• It is possible to use uncompressed .tar files as updates
• It is possible to use uncompressed .tar files in the header (header.tar, instead of header.tar.gz)

Update:
This has been implemented by a contributor in https://github.com/mendersoftware/mender-artifact/pull/150. The new acceptance criteria are:

• Update vendor dependencies in deployments service
• Make one end-to-end test that uploads and updates using an uncompressed artifact (reuse and tweak existing test probably)
• Add a section somewhere in the docs to explain that uncompressed artifacts are possible.

Reestimated to 5 due to help from outside.