A flag to display a list of available parameters

_From @sebawita on December 4, 2017 18:50_

### Bug Report or Feature Request (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [x] feature request
```

### Area
```
- [ ] devkit
- [x] schematics
```

### Desired functionality
Currently the only way to check the available parameters for a schematics is by looking into its json schema file.
It would be great to have a way in the CLI to display the list of params, like this:
```schematics @schematics:schematic --schema```

_Copied from original issue: angular/devkit#311_

Start by tracing the schematics CLI entry point and how it reads each schematic's JSON schema. Run the proposed `schematics @schematics:schematic --schema` command while checking the existing CLI behavior. Done means the flag displays the available parameters for a schematic without requiring users to inspect its schema file.

Written by the indexing model from the issue text.