Fix Placeholder parsing and improve os_specific

[MHendricks]

Checklist

• [x] I have read the CONTRIBUTING.md document
• [x] I formatted my changes with black
• [x] I linted my changes with flake8
• [x] I have added documentation regarding my changes where necessary
• [x] Any pre-existing tests continue to pass
• [x] Additional tests were made covering my changes

Types of Changes

• [x] Bugfix (change that fixes an issue)
• [ ] New Feature (change that adds functionality)
• [ ] Documentation Update (if none of the other choices apply)

Proposed Changes

Fixes exception raised if you use a uri that would hit a Placeholder object.

For the given configs:

$ hab dump . -t f
-------------------- Configs ---------------------
app
    hab.parsers.placeholder.Placeholder('app')
    |-- hab.parsers.config.Config('app/houdini19_py2')
    +-- hab.parsers.config.Config('app/maya2020')
default
    hab.parsers.config.Config('default')

trying to access a non-defined config in the same tree as app results in a error:

$ hab dump app/invalid
Traceback (most recent call last):
  File "C:\Program Files\Python39\lib\runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  ...
  File "C:\blur\dev\hab_\hab\parsers\flat_config.py", line 26, in _collect_values
    logger.debug("Loading node: {} inherits: {}".format(node.name, node.inherits))
AttributeError: 'Placeholder' object has no attribute 'inherits'

This fixes the error by enabling inherits for all Placeholder objects, and fixing a bug where it wouldn't fall back to default in that case. That code wasn't being tested, but is now.

Also added tests for the os_specific feature on site json files and for environment variable definitions.