[Certify model] Need to remove the possibility to certify model several times from different accounts.

[julia-aksyanova]

Severity:
major

Build info:
0505970

Precondition:
The system should have two or more users with the ZBCertificationCenter role.

Steps to reproduce:

1. Add to the system model-info with valid vid and pid (zblcli tx modelinfo add-model...).
2. Add test-result for this model: use the same vid and pid parameters (zblcli tx compliancetest add-test-result...).
3. Certify this model using one account with the ZBCertificationCenter role: use the same vid and pid parameters (zblcli tx compliance certify-model...).
4. Try to certify the same pair of vid and pid:
   a) using the same account with the ZBCertificationCenter role (from step 3);
   b) using another account with the ZBCertificationCenter role (not the same as in step 3).
5. Look at the result of the command execution.

Actual results:
The second certification command for the same pair of the vid and pid is successfully executed.

Expected results:
a) When the same account with the ZBCertificationCenter role (from step 3) is used at the second request, the confirmation message should appear: "override the existing certification info [y/N]: " (message should be discussed).
b) When another account with the ZBCertificationCenter role (not the same as in step 3) is used at the second request, error message should appear info the results: "Model with vid= and pid= is already certified on the ledger."(message should be discussed).

[ashcherbakov]

Fixed in #70