Support pre-check and post-check scripts

[gaborcsardi]

This is quite easy, and would solve a lot of issues.

These are the steps needed:

• [x] fix the Jenkins API client's HTTP library to allow multipart POST requests
• [x] fix the Jenkins API client to allow submitting file parameters
• [ ] update rhub-frontend to allow specifying the scripts in advanced mode
• [x] check if the rhub-frontend api puts the scripts in the message queue
• [x] update the rhub-backend templates to include the file parameters
• [x] update the rhub-backend Jenkins job creation code to pass the file parameters
• [ ] update rhub-backend linux script to pick up the JSON and run it
• [ ] update macoscheck to do the same
• [ ] update wincheck to do the same
• [ ] update solarischeck to do the same
• [ ] make sure the log output is reasonable
• [ ] update the rhub R package to be able to supply them to the check

[gaborcsardi]

Or maybe we could just allow running arbitrary scripts as in the pre-check and post-check phases? Scripts could also create files, if needed. Then we might as well have a complete sequence of overridable steps.

We can start with pre-check and post-check, though.

[gaborcsardi]

Need to fix the jenkins api package for this, to be able to upload files: https://github.com/silas/node-jenkins/issues/33

[gaborcsardi]

One question is what kind of shell (or language, really) we assume for the pre-check and post-check scripts. We can't just assume bash on windows, for example, that is not very useful.

Maybe we can go with bash on Unix and PowerShell on Windows, and the alternatives could be specified in dictionaries. E.g.:

[{ 
  unix: "ls",
  windows: "dir"
}.
...
]