Reduce Spinach parallelization to 2 nodes and increase RSpec's to 28 nodes

What does this MR do?

Re-balance 30 workers for rspec (28) and spinach (2), replacing !16675 (merged).

Are there points in the code the reviewer needs to double check?

See duration of rspecs and spinachs on https://gitlab.com/tnir/gitlab-ce/pipelines/16588965/builds:

• spinach average: 994 sec
• rspec average: 916 sec

All spinach tests (Pipeline #16588965)

job	duration/mm:ss
spinach-mysql 0 2	21:31
spinach-mysql 1 2	15:17
spinach-pg 0 2	15:38
spinach-pg 1 2	13:52

3 fastest rspec tests (Pipeline #16588965)

job	duration/mm:ss
rspec-mysql 8 28	09:12
rspec-mysql 22 28	10:28
rspec-pg 7 28	10:32

3 slowest rspec tests (Pipeline #16588965)

job	duration/mm:ss
rspec-mysql 3 28	34:23
rspec-mysql 20 28	28:54
rspec-mysql 14 28	23:24

Why was this MR needed?

Decrease overall test duration to save developers' time where the number of the rspec tests and increased.

Does this MR meet the acceptance criteria?

• [n/a] Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• [n/a] Tests added for this feature/bug
• Review
  • [n/a] Has been reviewed by UX
  • [n/a] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [n/a] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [n/a] Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

n/a

cf. !16675 (merged)