Respect entity namespace for entity registry

[balloob]

Description:

Fix entity registry not taking entity namespace into consideration when generating initial entity id.

Related issue (if applicable): fixes #

Example entry for configuration.yaml (if applicable):

light:
  platform: something
  entity_namespace: bla

Checklist:

• The code change is tested and works locally.

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.