Access environment variables through Host object instead of directly.

This makes it easier to test interactions with os.environ without
affecting or using actual environment variables in tests.

This CL doesn't change places where os.environ is used where there
is no Host object.

BUG=625846

Review-Url: https://codereview.chromium.org/2143123004
Cr-Commit-Position: refs/heads/master@{#405680}
15 files changed