Tagging Strategy with sub-packages

[dhermes]

I used the "question" label but this is more like an informative issue. Supersedes #2462.

Tag names will be as follows:

• {pkg}-0.x.y for releases to the google-cloud-{pkg} PyPI package, e.g. core-0.21.1, datastore-0.22.0, etc.
• 0.x.y for releases of the umbrella package (as we had already been doing)

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• For hand-rolled stable builds (i.e. gh-pages), we'll have a custom script which install from the latest version of each package on PyPI, rather than from the source tree (this may be a chicken-and-egg problem on tag builds that would both push docs and push to PyPI)
• We will be disabling have disabled (as of 1:47pm Pacific, Dec. 5, 2016) RTD stable builds: we can't have local filesystem deps on latest builds and then have PyPI deps on stable builds (RTD subprojects may help)