Update willis to use the ManifestCheckout class.
Update willis to use the ManifestCheckout class.

No reason to use the bare manifest class, and ManifestCheckout properly
sets up for include support.

BUG=None
TEST=manual

Change-Id: Ie64a40a26ca993c0143586a359dfc4d93c89dac4
Reviewed-on: https://gerrit.chromium.org/gerrit/36010
Commit-Ready: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Tested-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Reviewed-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>

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