PKI: Fix intermediate certificate idempotency

[markgoddard]

Fix idempotency of intermediate certificates

Prior to this change running the vault_pki role multiple times when generating an intermediate certificate could result in the following error:

    TASK [vault_pki : Set Intermediate as signed] **********************************
    fatal: [localhost]: FAILED! =>
      msg: |-
        The task includes an option with an undefined variable. The error was:
        'dict object' has no attribute 'data'
    
        The error appears to be in 'roles/vault_pki/tasks/intermediate.yml': line
        45, column 7, but may be elsewhere in the file depending on the exact
        syntax problem.
    
        The offending line appears to be:
    
            - name: "Set Intermediate as signed"
              ^ here

This change adds the same condition used in other tasks.

The tests have also been updated to cover PKI idempotency.