Users are stored individually, not in a single collection

Users were stored in collection 'users', that collection was updated from default data and
after the whole update process. In case of failure the collection misses all changes. The
issue is fixed by storing all users individually by their user_id, email and launchpad_id.

Fixes bug 1242588

Change-Id: Ie3d57ec7f3f18c4259dbd34cc8f29be45b06e2cd