wazo-auth return session with user_uuid not being a uuid

Given I one of the following endpoint from wazo-auth

• GET /users/<user_uuid>/emails/<email_uuid>/confirm

• POST /users/register

• GET /users/password/reset

Given I have in my DB a token with auth_id to being a uuid (ex: `wazo-auth`)
When I get sessions list
Then I have field `user_uuid=wazo-auth`

Result:
wazo-chatd fails to initialized because it fetches all sessions and try to insert uuid in its DB

Expected: value returned for field user_uuid in session is always a valid uuid or None value

Technical:

session use auth_id to populate user_uuid column and wazo-auth can use non uuid type for auth_id column

https://github.com/wazo-platform/wazo-auth/blob/master/wazo_auth/services/email.py#L96