disable extra vars

[evgeni]

we don't need them right now and it's confusing to the user

[Gauravtalreja1]

ACK, Tested this by manually updating /usr/bin/foremanctl and it works correctly to disable extra vars

Before Fix

# foremanctl features -h
usage: foremanctl features [-h] [-v] [-e EXTRA_VARS] [--list-enabled]

List all enabled and available features

options:
  -h, --help            show this help message and exit
  -v, --verbose         verbose output
  --list-enabled        List only enabled features

advanced arguments:
  -e EXTRA_VARS, --extra-vars EXTRA_VARS
                        set additional variables as key=value or YAML/JSON, if filename prepend with @

Parameters marked as (persisted) can be reset by --reset-<parameter-name>.

After Fix

# foremanctl features -h
usage: foremanctl features [-h] [-v] [--list-enabled]

List all enabled and available features

options:
  -h, --help      show this help message and exit
  -v, --verbose   verbose output
  --list-enabled  List only enabled features

Parameters marked as (persisted) can be reset by --reset-<parameter-name>.

I missed to post this comment before merge :)