"container image prepare" generates empty list if a tripleo env file is used

[Jan Provaznik]

If ~/.tripleo/environments directory exists, then the command to prepare docker_registry.yaml file returns an empty list of images.

Reproducer:
mkdir ~/puppet-modules
upload-puppet-modules -d ~/puppet-modules
openstack overcloud container image prepare --namespace trunk.registry.rdoproject.org/master --tag tripleo-ci-testing --env-file ~/docker_registry.yaml

Removing ~/.tripleo "solves" the issue and list of images is then generated.

I guess the problem will be in tripleoclient/v1/container_image.py where build_service_filter method takes into account only user's ~/.tripleo/ env files when getting enabled services.

This issue is painful for user experience as it generates an empty list w/o any warning so users may be completely lost what is happening.