Add passing tests for vm#configure_guest_customization_section
Fix empty preamble vars case

We were not handling the case when vars was not specified
in the configuration, resulting in a NilClass no method 'merge'
error.
Bump version, changelog
Better handling of bootstrap vars

as per @dcarley suggestion in https://github.com/alphagov/vcloud-core/pull/10/files#r10250941
Merge from master; fix version
Merge pull request #10 from alphagov/66653770_nil_vars_bugfix

[#66653770] Fix case where bootstrap vars is not specified.