Clean Docker images after builds

Clean Docker images after builds

Compared to the related Releng script, it only focuses on cleaning
images. It's considered out of scope to remove remaining docker
containers.

It doesn't force to remove images because base images (e.g. Alpine)
could be selected by other OPNFV projects too. It ensures O is
returned even if one image cannot be removed.

functest-core is removed at the end because all Functest containers
depend on it.

The script works well in a nominal building env. Any remaining
Functest images or containers must be removed by hand.

JIRA: FUNCTEST-898

Co-Authored-By: Cédric Ollivier <cedric.ollivier@orange.com>
Change-Id: I72d8214b711680ec7958af1d427b23dee6337bf6
Signed-off-by: Jose Lausuch <jalausuch@suse.com>
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>