[vpython] Allow user-supplied spec loader.
[vpython] Allow user-supplied spec loader.

BUG=chromium:741001
TEST=None

Change-Id: I919759b27be6846476efa52d405f4e7c6e7daeec
Reviewed-on: https://chromium-review.googlesource.com/639213
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Daniel Jacques <8400006a9338a0893dca0101ad49838955bd6b2c@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go