Add a common account to obtain all barbican's secret.

In current implementation [1], a registered vim's password is encoded by fernet, and the fernet key is saved in barbican as a secret. With barbican's default policy, only who created the secret can obtain the secret, which lead to a problem: registered vim can not be shared by other tenant.
But now we want to split the policy execution into mistral workflow, in this case, we want to obtain all vim's password to do some VNF LCM operation.
This BP's goal is to support a shared account to obtain barbican's secrets.

1. https://blueprints.launchpad.net/tacker/+spec/encryption-with-barbican