create volume backup failed when enable TLS

[Jie Li]

Condition：
1.Cinder backup driver: swift
2.backup_swift_auth mode: single_user
3.Cinder service and swift service are enable TLS
4.cinder.conf is
  backup_swift_auth = single_user

We create volume backup failed when we enable TLS.
The reason is that we can't set the swift endpoint type, and default is PUBLIC endpoint type[1].
For more security, we should set the swift endpoint type is internal when the OpenStack enable TLS.
So we should let the operator set the swift endpoint type.

[1]:https://github.com/openstack/python-swiftclient/blob/master/swiftclient/client.py#L623

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/742136