[WIP] scripts: Add script that performs entire release workflow

[jpbetz]

We should test out this script very carefully for a couple actual releases before giving it the stamp of approval, but I wanted to post it up in a PR so it's available.

The steps it takes are almost identical to those taken by the 'etcd-release-runbook' (https://goo.gl/Gxwysq) workflow, with the following differences:

• Not all steps have been implemented yet. In particularly the signing step is not yet implemented.
• The script performs a variety of additional checks in an attempt to avoid mistakes, e.g.: "does the current go version match the one in .travis.yml? Are gpg keys set up properly? Do the version/version.go, git tag version and desired go release version all match? ..."

Please try it out and send me feedback.

[gyuho]

@jpbetz Let's try this script in our next April patch release, before merge and backport. Thanks.

[codecov-io]

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[gyuho]

Merging to try this today. Thanks!