Introduce a Parse function that takes care of expanding vars while parsing
the DEPS file.

It wraps Exec and exec calls, and supports deferring the expansion until
later, so gclient flatten gets access to the unexpanded version.

Bug: 821199
Change-Id: I943b021cc4474c9cda67b3816b841dd8ada3f5b2
Reviewed-on: https://chromium-review.googlesource.com/973749
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>