cli: git-tag

[tiborsimko]

Introduce git-tag command that for each component would create git tag corresponding to the release.

Example: We would like to release r-commons and r-client, so we prepare release commit, create tag, and upload to pypi:

$ reana-dev git-create-release-commit -c r-commons -c r-client
$ reana-dev git-tag -c r-commons -c r-client
$ reana-dev release-pypi -c r-commons -c r-client

The new command git-tag would do:

• for each component passed as argument, do:
• check whether the latest commit is release: vX.Y.Z one; abort if not (and advice to use r-dev git-create-release-commit command)
• detect version number of the component by reading compontent/version.py -> X.Y.ZaN
• do git tag vX.Y.ZaN

The final status is that the component should have the last commit the usual release: vX.Y.Z commit and the git tag corresponding to the release commit version.

P.S. We do this separately, and on locally checked out branches by the integrator, because we shall be releasing packages on PyPI, but if something fails during this process (e.g. Python trove classifiers for PyPI are wrong), we would still be able to amend it before pushing the release and the tag to upstream on GitHub.