preprov creds get_admin_creds can return project or system scoped cred

[Sam Morrison]

if you have following accounts.yaml

- project_domain_id: default
  project_domain_name: default
  project_name: tempest-admin
  roles:
  - admin
  user_domain_id: default
  user_domain_name: default
  username: tempest-admin
  password: test
- roles:
  - admin
  system: true
  user_domain_id: default
  user_domain_name: default
  username: tempest-system-admin
  password: test
....

When running eg. tempest.api.network.admin.test_external_network_extension.ExternalNetworksTestJSON.test_delete_external_networks_with_floating_ip

The admin credential that gets set up could be either the system-scoped admin or the project scoped admin.
It should be the project scoped admin.

This test then fails with:
Details: {'type': 'HTTPBadRequest', 'message': 'Running without keystone AuthN requires that tenant_id is specified', 'detail': ''}
When tempest attempts to create a network. (create network is project scoped in neutron policy)

To fix this in my env I have just got get_admin_creds() to call get_project_admin_creds()