Improve version handling

[ark3]

Goals

• Telepresence knows whether it is running from a released artifact or from source
• Telepresence warns the user when environment variables override the version of the proxy image (and the local image for container method)
• Telepresence reports both the original (built-in) version and override version info when an environment override is used
• Tagged release commits and unreleased commits have a computable version number that the deploy script can use to mark the artifacts it generates (Telepresence itself, images, deb/rpm packages, etc.)
• When running a released codebase, Telepresence reports a release version string (e.g., 0.88) and uses the same version for images
• When running installed from a non-release commit, Telepresence reports a dev version string (e.g., 0.88+3.gFEED1BEEF.dirty) and uses the same version for images
• When running from a developer's Git checkout, Telepresence reports a dev version string but uses the prior release's images (0.88)

Plan

• Remove all hard-coded references to the version number.
  • Such references are already catalogued in .bumpversion.cfg
  • docs/build-website.py will need special handling
• Remove Bumpversion
• Add Versioneer
• Update the deploy.py script so that unique dev version numbers are used on untagged builds (as opposed to re-using the prior release's version number, which is what happens right now)
• Update the release process documentation
  • Manually compute the next version
  • Call towncrier with the new version number as before
  • Tag the repo manually with the new version number. This replaces the current bumpversion step

Versioneer automatically takes care of reporting the appropriate type of version number from a source tree, from an installed deployment, etc. It computes the version based on the SCM state in dev. When using setuptools to deploy/install, it replaces the version computation code with a hard-coded value. It may be some work to figure out which case is in play; need to investigate.