[design] Introduce DCOS_CLI_EXPERIMENTAL_AUTOINSTALL_PLUGINS
[design] Introduce DCOS_CLI_EXPERIMENTAL_AUTOINSTALL_PLUGINS

Currently, default plugins are installed on cluster setup unless the
`--no-plugin` option is passed. This updates the design to have this
as an experimental opt-in feature through an environment variable.

https://jira.mesosphere.com/browse/DCOS_OSS-4186

dcos/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli,kensipe/dcos-cli,kensipe/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,dcos/dcos-cli