twister: allow list input for set config fields

Twister configuration files have a few fields defined with the "set"
type. The set needs to be specified as a space-separated list of
entries, for example:

```yaml
plaform_allow: boardx boardy
```

This patch allows users to write these fields as YAML lists, for
example:

```yaml
platform_allow:
  - boardx
  - boardy
```

The list gets converted to a Python set, therfore the same exact
behavior is kept in Twister.

In some cases like in the example above, using lists allows to have more
readable configuration files, specially if the list is long. It also
helps on diffs, provided we have one item per line.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>