Refactorization for run_existing end-point

[Tansito]

Summary

Continuing with #1240 and #1243 this is a proposal to move views and services logic to serializers.

The idea behind this refactorization is:

• Use more serializers in the different views
• Concentrate the access to the models in the serializers
• We move from a non-standard class: service, to a first-class member for DRF: serializers

This PR concentrates the work in the run_existing end-point.

Details and comments

• Two new serializers are created:
  • RunExistingProgramSerializer: it will manage the input from the request for the rest of the logic
  • RunExistingJobSerializer: specific logic for Job in this end-point.
• Models optimization:
  • Field created is not editable
  • Fixed __str__ method in JobConfig model
  • Fixed default value for Job.status. We were using QUEUE by default.

The rest changes are minor fixes:

• Removed Custom exceptions. They are not working properly. I will take a look later as part of Client + gateway: improve error handling  #1264
• From Fix ExistingProgram serializer #1245 fixed the type of build_env_variables.arguments

[Tansito]

I didn't remove services or review if I can reuse a serializer in other end-point just to avoid to add more changes to the PR. Probably in the run end-point PR I'm going to be able to reuse some serializers but I will take a look later on.

[IceKhan13]

Looks clean! let's give it a go