Add idempotentcy to pihole role.

Added:

• pihole_idempotent. This will force role to be idempotent each run. Classic
  behavior is false (not idempotent); but may now be optionally toggled on.

Changed:

• Added Default group to blocklist defaults to conform to pihole defaults.
• Added remove_default_group to pihole_clients (backwards compatible).
  Hosts toggling this flag will be removed from the Default group on role
  application.
• Adding clients moved to separate add_clients file enabling management of
  client and removal of Default group in one loop.
• Databases may now be removed on role application to enable idempotent runs.