[gitlab-ci] Various fixes for scheme

[AKorezin]

Why did I create this PR

1. I want to fix definitions for job variables.
   But to after fixing variables I've found that only top level variables support object type.
2. So to test this I have to create positive and negative tests.
3. To create negative tests I have to fix gitlab-ci scheme.

PR details

Fix gitlab-ci to pass schema test to support negative tests
Add secrets definition with full inner object structure.

Create global variables definition with additional object property for prefill variables.

# This is valid
variables:
  VARIABLE:
    value: DEFAULT
    description: "Variable test"

Remove additional object property from job variables, because objects are only supported at top level.
Ref: #1504

# This is invalid
test_job:
  script:
    - true
  variables:
    VARIABLE:
      value: DEFAULT
      description: "Variable test"

Add positive test for variables.
Add negative test for variables.

[madskristensen]

Thanks