Demonstrate the new workflow for resolving containers.
1. First call Manifest().
2. Get container SourceSpecs from manifest struct.
3. Resolve them.
4. Serialize() with resolved container specs.
The changes in the test manifests are just the information about the
container sources (was a slice but is now a map) and the actual manifest
object isn't affected.
The TestDistro_Manifest test in distro_test_common is adapted
accordingly as well.
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