Rename 'sid' to 'user' in access rules and sec services

Rename 'sid' (secure identifier) to 'user' for better understanding by user.
Changed parts:
- db entities
- db migration
- share access rule type
- security services
- tempest tests

Change-Id: I527064508f13bd4dc658746970c2606098199c1c
Merge "Rename 'sid' to 'user' in access rules and sec services"