Refactor `GitlabCiYamlProcessor`

Currently GitlabCiYamlProcessor have much more than a single responsibility. It:

• validates entire .gitlab-ci.yml
• validates jobs
• validates stages
• provides some default values
• does initial parsing
• builds jobs from configuration
• selects jobs for stage and ref
• normalizes script

I think it may be a good idea to extract some classes.

We can create classes like Ci::Config, Ci::Config::Validator(::Job, ::Global, ::Stage), Ci::Config::Builder, Ci::Config::Parser(::Yaml) etc. With this approach it will be a lot easier to extend it, refactor, provide additional configuration formats, and so on.

What do you think @ayufan?