Snapshots instead of nigtlies

[tsg]

• Snapshot/nigtlies no longer contain the date in the file names
• We will publish now only one SNAPSHOT, the latest one
• Inside all packages, there is a .build_hash.txt file containing the git hash so we can track the moment when the snapshot was created.
• The default is to add -SNAPSHOT, you can disable it by calling dev-tools/deploy --no-snapshot or by make SNAPSHOT=no in dev-tools/packer

In addition, this PR simplifies a bit the packaging process:

• No longer use xgo, rather just call the docker commands directly. This
  is not a lot more complex and it's easier to modify the command (e.g add
  another env variable) when needed.
• The logic to copy nightlies to latest is no longer needed
• the deb_version and rpm_version are replaced with a sed that replaces -
  with ~ in the platform scripts
• No longer use docker cp to copy the artifacts over, we already have a
  shared volume and that's faster.

[andrewkroh]

Should this also be changed to be the git commit hash?

[andrewkroh]

Jenkins beats-package job will need to be updated to remove the following logic from the build command

# In file names, $BUILDID is replaced with "latest" to create that latest artifacts for S3.
export BUILDID=$(git rev-parse --short HEAD)
ver=$(python ./dev-tools/get_version | sed "s/-\w\+/-nightly${BUILDID}/")
python ./dev-tools/set_version $ver
python ./dev-tools/deploy