Optimize fields and calls

[Tansito]

From #1285 @akihikokuroda was doing a good optimization passing standard JSON instead of the string. I wanted to propose a set of optimizations after review his pull request:

• Pass arguments as standard JSON similar to config in that pull request
• Pass env_vars as standard JSON similar to config in that pull request

The reason behind these proposals is that we are encrypting this data anyway so what we are storing is the string of the encryption.We could receive in the API the JSON, parse it with the JSONField serializer to verify that is a valid JSON (something that we are not doing right now) and store it after encrypt it.

Apart from this. We are storing arguments in two places, in arguments field and env_vars field after being encrypted. I propose two approach:

• Remove arguments field from Job (easiest one)
• Remove arguments from env_vars and store them separately

[Tansito]

I put on-hold this conversation until we solve the problem with JSON and content-type multipart: #1286 (comment)