Add GET /api/job/{jobid}/files/{path} route

[sverhoeven]

Fixes #44

Also adds GET /api/job/{jobid}/stderr route.

To test

1. Execute job as described at https://github.com/i-VRESSE/bartender/tree/44-host-job-files#word-count-example
2. Try out the GET /api/job/{jobid}/files/{path} route
   • with path=returncode should return same content as /tmp/jobs/1/returncode file on filesystem.
3. Try out the GET /api/job/{jobid}/stdout route
   • should return same content as /tmp/jobs/1/stdout.txt file on filesystem.
4. Try out the GET /api/job/{jobid}/stderr route
   • should return same content as /tmp/jobs/1/stderr.txt file on filesystem.
5. Test nested paths
   1. on filesystem run mkdir /tmp/jobs/1/output && echo foo > /tmp/jobs/1/output/somefile.txt
   2. Try GET /api/job/{jobid}/files/output/somefile.txt
   3. Should return foo as content

[sverhoeven]

Thanks for reviewing