Rename `CloudEnvironment` -> `DaskKubernetesEnvironment`

[joshmeek]

Couple of changes here:

1. Rename the environment
2. Allow it to be slightly more customizable. A good first step is allowing the # of min/max workers to be specified

[cicdw]

Just putting this here for lack of a better place: we should also remove the unnecessary layers from the environments/ submodule file structure. For example, the remote environment should live in src/prefect/environments/execution/remote.py, not a deeper directory.