Added execution engine configs

[applejag]

• I've added a new note in the CHANGELOG.md file, according to docs:
  https://iver-wharf.github.io/#/development/changelogs/writing-changelogs

Summary

• Added config:

  ci:
    # old way:
    triggerUrl: http://jenkins.local/trigger
    triggerToken: changeit
  
    # new way:
    engine:
      id: jenkins # defaults to "primary"
      name: Jenkins # defaults to "Primary"
      url: http://jenkins.local/trigger # defaults to ci.triggerUrl
      token: changeit # defaults to ci.triggerToken
  
    engine2:
      id: wharf-cmd # defaults to "secondary"
      name: wharf-cmd # defaults to "Secondary"
      url: http://wharf-cmd-provisioner/api/trigger
      token: changeit

• Added response models for engines

• Added endpoint GET /api/engine

• Added query parameter ?engine= to "start build" endpoint

• Added engine ID to build table

• Added engine to build responses via engineLookup

• Added a new problem type: /prob/api/engine/no-default

Motivation

Allows for a more natural transition over to wharf-cmd instead of Jenkins, as the user can specify what execution engine they want to use on a per-build basis.

With such, we can have Jenkins as the primary and default engine until wharf-cmd is feature-complete with our Jenkins implementation. Then we could switch so wharf-cmd is the primary engine and keep Jenkins as a secondary to allow using that one if the wharf-cmd implementation fails.

The config design is a bit weird. The reason is that Viper, the config library we're using via wharf-core, doesn't support arbirary names or indices for environment variables. It can only perform lookups on fields it already recognizes. See #132 for the initial YAML config design using a map[string]Engine. This config design limits us for now to only having a primary and a secondary engine, but can be expanded further by adding ci.engine3, ci.engine4, etc. From the REST API perspective, it's just a list of engine definitions.

After merge

• Create PR to iver-wharf.github.io for docs on new problem type: /prob/api/engine/no-default

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Closes #132

[applejag]

I pushed two minor bug fixes. Hope you're OK with me merging without you review again.