Previously updating the credentials would delete values from the existing crednetials if they weren't also present in the update. This adds a method for merging credentials to the KeyVaultCredentials model and adds tests to both the cloud provider and model.
The tenant ID should be hashed and used as the key for the JSON blob of
relevant creds for any given tenant. Azure CSP interface methods that
need to source creds should call the internal `_source_creds` method,
either with a `tenant_id` or no parameters. That method will source the
creds. If a tenant ID is provided, it will source them from the Key
Vault. If not provided, it will return the default creds for the app
registration in the home tenant.
