Use now() as default timestamp for MySQL

What does this MR do?

Use now() as default timestamp for MySQL

Why was this MR needed?

This patch was originally from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13560 which tried to stop MySQL from using CURRENT_TIMESTAMP. However, as it seems, on some MySQL version (e.g. 4.7) and under some configuration, 0 is not allowed.

We never intended to use the default value, so any value should be fine. However, now() is picked because there's some weird issue in factory_girl along with MySQL, which stops Rails auto-generating timestamps, causing it use the default. Use now() could somehow "fix" this issue when it happened.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/36405#note_40120201
• https://gitlab.com/gitlab-org/gitlab-ce/issues/35376#note_40588738
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12841#note_40829586