Allow JSON parameters in environment files

Currently users can specify:
Parameters:
  foo: "[ \"a\", \"stringified\", \"JSON\", \"list\" ]"

This change allows:
Parameters:
  foo:
  - a
  - JSON
  - list

Change-Id: Ie1616ec4a215f62bc8e06ae2588f953246870b37
Closes-Bug: #1342487
Merge "Allow JSON parameters in environment files"