Using the new --list-dependencies flag, one can query dependencies for a
given filter. For example:

./build.py --list-dependencies heat swift
{'base': [{'openstack-base': [{'heat-base': ['heat-engine',
                                             'heat-api-cfn',
                                             'heat-api']},
                              {'swift-base': ['swift-object',
                                              'swift-proxy-server',
                                              'swift-container',
                                              'swift-rsyncd',
                                              'swift-account']}]}]}

Also added --list-images to list all available images.

Change-Id: I1797e32e32705182a763f53329eeb5c4a361abec
Implements: blueprint images-dependency-tree-cli