Switch to entry points for registering the env subcommand.

This was done as a reference implementation for https://github.com/conda/conda/pull/993.  It should not be merged until 993 is merged and released.
This function is pretty useful and should probably be in `conda.cli.common` as a way for nested sub commands to default to the help parameter.  This makes a call to `conda env` look as if it were actually called `conda env --help`.  The same should be done for `conda build` and any other commands that want to default to displaying the help.