Raise create work flow isolation back to Serializable

[ryli17]

Description

After PR #346 merged, raise back isolation level from repeatable read to serializable.
Also add create unique index ddl to tinkerbell-init.sql.

How Has This Been Tested?

• Manually

How are existing users impacted? What migration steps/scripts do we need?

• No

Checklist:

I have:

• updated the documentation and/or roadmap (if required)
• added unit or e2e tests
• provided instructions on how to upgrade

[codecov]

Codecov Report

Merging #348 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #348   +/-   ##
=======================================
  Coverage   23.79%   23.79%           
=======================================
  Files          14       14           
  Lines        1244     1244           
=======================================
  Hits          296      296           
  Misses        928      928           
  Partials       20       20           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3b230d5...084b6cc. Read the comment docs.