Add support for ci_jwt_signing_key Rails secret

This secret is RSA private key used to sign CI_JOB_JWT. The key can be
provided with `gitlab-secrets.json` or `gitlab.rb`, and will be
generated if not provided.

Related to gitlab-org/gitlab#214607
21 jobs for 214607-ci-jwt-signing-key in 112 minutes and 37 seconds (queued for 1106 minutes and 51 seconds)
Status Job ID Name Coverage
  Prepare
passed #547891730
check_docs_lint

00:01:34

passed #547891729
danger-review

00:00:46

 
  Review
manual #547891738
allowed to fail manual
review-docs-cleanup
manual #547891737
allowed to fail manual
review-docs-deploy
passed #549515472
review_eks

00:08:16

passed #547891732
review_gke

00:24:23

passed #547891736
manual
stop_review_eks

00:00:33

passed #547891735
manual
stop_review_gke

00:00:40

failed #547891734
review_eks

00:16:00

 
  Specs
passed #547891745
review_specs_eks

00:07:05

passed #547891739
review_specs_gke

00:10:17

passed #547891747
specs_without_cluster

00:06:47

 
  Qa
passed #547891751
debug_review

00:01:25

failed #549564871
allowed to fail
qa_eks

00:56:01

failed #549567156
allowed to fail
qa_gke

01:00:01

passed #547891748
review_helm_test

00:00:28

failed #547891755
allowed to fail
qa_eks

00:55:31

failed #547891753
allowed to fail
qa_gke

01:00:00

 
  Package
passed #547891758
Validate 1.12.9

00:00:49

passed #547891759
Validate 1.16.0

00:00:58

passed #547891757
lint_package

00:00:42

 
Name Stage Failure
failed
qa_eks Qa
      # ./qa/scenario/template.rb:8:in `perform'
# ./qa/scenario/template.rb:33:in `perform'
# ./qa/scenario/template.rb:10:in `block in perform'
Running after_script
Uploading artifacts for failed job
Uploading artifacts...
./gitlab-qa-run-*: found 119 matching files

Uploading artifacts to coordinator... ok
id=549564871 responseStatus=201 Created token=hox1Hx6G
ERROR: Job failed: exit code 1
failed
qa_gke Qa The script exceeded the maximum execution time set for the job
      
1st Try error in ./qa/specs/features/browser_ui/3_create/web_ide/review_merge_request_spec.rb:23:
Running after_script
WARNING: Failed to inspect build container ee8a0e0311d7a6c24f7fb3f01410e6d4600838b3746886ccb47ac64b3050b327 context deadline exceeded (docker_command.go:77:0s)
Authenticating with credentials from job payload (GitLab Registry)
Pulling docker image registry.gitlab.com/gitlab-org/gitlab-omnibus-builder/ruby_docker ...
Uploading artifacts for failed job
Pulling docker image gitlab/gitlab-runner-helper:x86_64-6c8c540f ...
ERROR: Job failed: execution took longer than 1h0m0s seconds