syndic_finger option

Created a new syndic_finger option to distinguish it from the minion
option.

Refactored the finger check a little bit for DRY.

If a syndic is authing and the syndic_finger option is set, it will
use that. Otherwise, a syndic will fall back to master_finger in
order to preserve backwards compatability. Functionality for minions
is unchanged.

Closes #8866.