Easily resolve HAB_FREEZE when inside of that config

[MHendricks]

This allows you to easily get a hab Config object for the hab config you are currently running inside of.

Currently you have to run this code:

import os
import hab

default_uri = "a/uri"
freeze = os.getenv('HAB_FREEZE')
uri = os.getenv('HAB_URI', default_uri)
resolver = hab.Resolver.instance()
if freeze:
    cfg = hab.parsers.UnfrozenConfig(freeze, resolver)
else:
    cfg = resolver.resolve(uri)

This Pull Request allows you to replace that code with.

resolver = hab.Resolver.instance()
cfg = resolver.resolve("a/uri", from_env=True)

Checklist

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

Types of Changes

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

Proposed Changes