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.

This decouples Ci::Config to provide a common interface for handling
user configuration files.