[docker] Use image IDs instead of tags

If we use tags, when there are remaining images without
the tag <none> it fails and get the following error:

Removing docker image opnfv/functest:<none>...
Error response from daemon: Error parsing reference:
"opnfv/functest:<none>" is not a valid repository/tag

By using IDs we make sure we delete the images.

Change-Id: Ib336dc73168bac909c42dd4a9681ef07d477dc7b
Signed-off-by: jose.lausuch <jose.lausuch@ericsson.com>