add a more strict schema

[stolarczyk]

right now the config schema just requires compute_packages section to be present and asserts the structure of the adapters and compute_packages.
Do we want to add more restrictions? For example:

• require compute_packages.default to be defined
• require submission_template and submission_command for every package?

compute_packages:
  default: <-- require?
    submission_template: templates/local_template.sub  <-- require?
    submission_command: sh  <-- require?

Originally posted by @stolarczyk in #54 (comment)