In order to implement https://gitlab.com/gitlab-org/gitlab-ee/issues/10179
we need several modifications on the CI config file. We are
adding a new ports section in the default Image object.
Each of these ports will accept: number, protocol and name.
By default this new configuration will be only enabled in
the Web IDE config file.
Introduce `default:` configuration entry setting that makes it possible
to configure a default value of an entry, what overrides class-level
`def self.default` value.
This commit refactors only/except policies so that these policies could
be self-contained. This also adds some changes to YAML configuration
library to provide more context to default entry value fabrication
process.
