This is a role to abstract removal of tags from registries, which is
an operation that practically has to be done via the registry API.
This implements removing tags from the quay and docker API's.
For the common case of working with a repository like
"quay.io/org/project" there is minimal configuration. However, if you
run a private repository, this is flexible with a few extra variables
to tell the role to use the quay API but your own URL.
By default it clears out old tags from the Zuul promote pipeline.
However if you set registry_tag_remove_tag it will only remove that
one tag.
This is inspired by the current work done in promote-docker-image
role.
Change-Id: I7f2d9d00024e34451e2d20b2c2f8171ecd151943