Add an AutoHide value for config's hidden.
Add an AutoHide value for config's hidden.

It will cause the item to be hidden if it's in the 'default' state (i.e. its data
is the empty_val), and non-hidden otherwise. This cuts out a lot of cruft from
the various expectations, as well as no longer omitting non-critical values in
real recipe runs.

R=agable@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/23536073

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@224234 0039d316-1c4b-4281-b951-d872f2087c98

luci/recipes-py,luci/recipes-py,shishkander/recipes-py,shishkander/recipes-py