build:env:create --db-only flag for initial environment creation

[daggerhart]

Working on a site that has a very large file system of Drupal media. The multidevs created don't always need the entire filesystem copied over, because we can easily enable stage_file_proxy on the multidev.

The \Pantheon\TerminusBuildTools\Commands\EnvCreateCommand::createBuildEnv() method already allows fo an option called --db-only, and the \Pantheon\Terminus\Collections\Environments::create() method supports options for no-db and no-files.

At a glance, it seems trivial to allow site owners to create an environment without copying the filesystem. By interpretting the EnvCreateCommand::createBuildEnv() --db-only flag to the Environments::create() no-db option.

Is there any reason not to do this?