Implemented spawn config validation

[victormlg]

No description provided.

[olehermanse]

It's a bit hard to follow the code without seeing the bigger picture, please add:

• --validate option which validates the config and won't spawn in the future when that is implemented.
• Some shell tests which use the --validate option and prove that it's passing and failing in the right situations.
• Some explanation and examples for the feature in the README. Clearly mark it as work in progress.

[olehermanse]

count should be moved to group, not be in template

[victormlg]

Templates is not a real key in the final config, all the names defined in templates get expanded before analyzing the config. Ex:

templates:
  mycfengine: 
    version: 3.27.0

groups:
  - myhub:
      role: hub
      cfengine: mycfengine

Gets turned into:

groups:
  - myhub:
      role: hub
      cfengine: 
        version: 3.27.0

Also, I believe it makes sense to have "count" inside "spawn", Ex:

groups:
 - client1:
      role: client
      source:
        count: 4
        mode: spawn
        spawn:
          provider: vagrant
          vagrant:
            box: ubuntu/focal64
 
  - client2:
      role: client
      source:
        # count: 4. Here count doesn't make sense, because we have saved hosts
        mode: save
        hosts: [ 8.8.8.8 ]