Don't do any caching logic if we are only serving payloads.
Don't do any caching logic if we are only serving payloads.

Currently, if we are only supposed to be serving images, the devserver
-at startup- might still try to clean its cache. This is unintended and
incorrect. If we are only supposed to be serving payloads, we shouldn't
have any additional side-effects.

BUG=chromium-os:34768
TEST=Ran w/ w/out archive_dir also ran unittests + pylint + --clear_cache w |
w/out data. A canary with vmtests and a paladin.

Change-Id: I7f97f9748f7eecac9da443b5881adac443f00fcd
Reviewed-on: https://gerrit.chromium.org/gerrit/39510
Commit-Ready: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Tested-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>

mgireesh05/dev-util,coreos/dev-util,coreos/dev-util,mgireesh05/dev-util,marineam/coreos-dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,coreos/dev-util