"bot_update" currently passes a full "gclient" specification as a
command-line argument to "bot_update.py". This file contains all sorts
of junk that clutters the command-line, and revealed a batch file
escaping bug that seems to be present in the recipe engine. Change this
so that "bot_update" passes the spec as file content instead of a
command-line flag.

Also, "bot_update" apparently called "--spec", which optparse kindly
recognized as the real flag, "--specs". This has always(?) been a bug,
and is no longer a problem.

BUG=chromium:738348
TEST=expectations

Change-Id: I5324b94dd5ef78503e7cfdc28b5fde8f434e4359
Reviewed-on: https://chromium-review.googlesource.com/558207Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Daniel Jacques <dnj@chromium.org>