Configure -once via config.hcl

### Envconsul version
envconsul v0.10.0 (1835ce90)

### Configuration
```hcl
once = true
```

### Command
```shell
envconsul -config /etc/envconsul.hcl
```

### Debug output
```
2020/10/28 09:56:29 [ERR] (cli) from file: /etc/envconsul.hcl: mapstructure decode failed: 1 error(s) decoding:

* '' has invalid keys: once
```

### Expected behavior
What should have happened?

envconsul should start the same way it would do as with "-once" parameter

### Actual behavior
What actually happened?

envconsul crash with the reason that "once" is not a valid config entry.

### Steps to reproduce
1. create a config where once = true
2. start envconsul with this config
3. envconsul crashes

### References
Are there any other GitHub issues (open or closed) that should be linked here?
For example:

- GH-1234
- ...