Fix config conversion in image cog

[Jackenmen]

Type

• Bugfix
• Enhancement
• New feature

Description of the changes

Fixes this error during conversion from pre-3.1 releases:

[2020-02-27 20:50:52] [ERROR] red: Package loading failed
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/redbot/core/core_commands.py", line 114, in _load
    await bot.load_extension(spec)
  File "/usr/local/lib/python3.8/dist-packages/redbot/core/bot.py", line 870, in load_extension
    await lib.setup(self)
  File "/usr/local/lib/python3.8/dist-packages/redbot/cogs/image/init.py", line 6, in setup
    await cog.initialize()
  File "/usr/local/lib/python3.8/dist-packages/redbot/cogs/image/image.py", line 33, in initialize
    await self.bot.set_shared_api_tokens(client_id=imgur_token)
TypeError: set_shared_api_tokens() missing 1 required positional argument: 'service_name'