Add unit tests for the PushMetadataAssembler

[phavekes]

This issue is imported from pivotal - Originaly created at May 28, 2018 by Michiel Kodde

This important piece of code (responsible for mapping the pushed metadata to EB) is not covered by unit or integration tests.

[baszoetekouw]

Let's start by adding unit tests. Integration tests should probably be taken up in a devconf setting where Manage is also available.