influx restore requires a single positional argument even when --input is specified

[danxmoran]

Steps to reproduce:

1. Create a backup using influx backup
2. Attempt to restore the backup using influx restore --input <path-to-the-backup>, as shown in our public docs

Expected behavior:
CLI attempts to restore the backup.

Actual behavior:
Error: must specify path to backup directory

---

The CLI is currently coded to require a single positional arg, for use as the input path. The --input option is effectively useless because of this.

To fix:

1. Don't require a positional arg when --input is given
2. Mark --input as deprecated / log a warning when it's used