This PR updates Lexicon dependency to the latest version available, 3.0.6, for every lexicon-based DNS plugins. It updates also the provider construction to use the new ConfigResolverobject, and to remove the legacy configuration process.
* Added DNS based authenticator plugin for Linode
* Added linode plugin to docs
* Added Dockerfile
* Added .gitignore and readthedocs.org.requirements.txt
* Updated default_propagation_seconds
* Updated according to changes requested
* Bump version to 0.26.0
* Advertise our packages work on Python 3.7.
