Improve json schema with better null handling and other fixes

[cpuguy83]

Update json schema to be more lenient on types

Before this change the schema validation is very noisy, particularly for
yaml where you can provide a value of nothing to represent `null`.
e.g.

```
args:
  foo:
  bar:
```

This applies to more than just args, its all over. Every map and every
struct type.

This change allows null values in the jsonschema for fields or values
that are not required.

Additionally it updates args to support integers which are just
converted to strings during unmarshal.

Allow empty test.steps and bases