nest `exec` plugin assertions under `assert` field

[jaypipes]

I'm aligning all of the plugin assertion fields with a name of assert. This commit changes the first plugin, exec to nest the exit_code, err and out assertion structs under an assert field object in the Spec.

So, instead of this:

tests:
 - exec: echo cat
   out:
     is: cat

you now do this:

tests:
 - exec: echo cat
   assert:
     out:
       is: cat

I'm changing the http plugin's assertions block from response to assert and the kube plugin's assertions block from kube.assert to just assert in followup PRs in those repositories.