feat(deisctl): check for required configuration on platform start

This PR adds checks for required configuration on `deisctl start platform`.

 * If `/deis/platform/domain` is missing, we print an error message and exit since the controller won't start
 * If `/deis/platform/sshPrivateKey` is missing, we print a warning and continue, since only `deis run` will be affected

Output on missing `/deis/platform/domain`:

```console
$ deisctl start platform
Error: Missing platform domain, use:
deisctl config platform set domain=<your-domain>
$
```

Output on missing `/deis/platform/sshPrivateKey`:

```console
$ deisctl start platform
Warning: Missing sshPrivateKey, "deis run" will be unavailable. Use:
deisctl config platform set sshPrivateKey=<path-to-key>
● ▴ ■
■ ● ▴ Starting Deis...
▴ ■ ●
```