boto3 not installed when custom backend defined

When using a custom (named) backend, like this...

glance_additional_stores:
  - swift
  - http
  - cinder
  - name: minio
    type: s3

...the dependencies of that backend may not be installed, as the logic expects a list and not a dict.

https://github.com/openstack/openstack-ansible-os_glance/blob/152abb5f6c680553bfad9bfee739d07d8767aeb3/defaults/main.yml#L264

 - "{{ ('s3' in _glance_available_stores) | ternary('boto3', '') }}"

In our case, though, _glance_available_stores would have:

[swift,http,cinder,minio:s3]

This will result in glance-api not properly starting since boto3 is not installed. A quick fix would be to enable 's3' by itself, but defeats the purpose.