Hotfix/open option no default

- Fixing option --open in command `start/restart` not null;
- If the default value of the option at a command is null, you can set the placeholder that will be the value displayed by default.

**before:**

```bash
$ azk help start
Usage: $ azk [options] start [options] [system]

Starts an instance of the system(s)

options:

  --verbose, -v, -vv        Sets the level of detail (default: false) - multiples supported
  --reprovision, -R         Force the provisioning actions before starting an instance (default: false)
  --open="", -o  Open a url of default system in the preferred application
```

**after:**

```bash
$ azk help start
Usage: $ azk [options] start [options] [system]

Starts an instance of the system(s)

options:

  --verbose, -v, -vv        Sets the level of detail (default: false) - multiples supported
  --reprovision, -R         Force the provisioning actions before starting an instance (default: false)
  --open="application", -o  Open a url of default system in the preferred application
```