validation_pool.py: base sleep interval on max timeout.
validation_pool.py: base sleep interval on max timeout.

Previous code just forced 30s regardless of the timeout interval.

This however made the tests invalidly run for longer than needed
for related tests.  As such maintain the 20% sleep interval, just
calculated from actual max timeout.

This brings test runtime down from ~35s to 11s.

BUG=chromium-os:25945
TEST=python validation_pool_unittest.py

Change-Id: Ide2ef008e337740145fc78b44c3568b563ffb4f8
Reviewed-on: https://gerrit.chromium.org/gerrit/16308
Commit-Ready: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Tested-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>

chadversary/chromiumos.chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,chadversary/chromiumos.chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,zhang0137/chromite,coreos/chromite