Exposes InputApi.rietveld and deprecate host_url.

This is useful since it places authentication at a single place.

BUG=
TEST=

Review URL: http://codereview.chromium.org/6825008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@81019 0039d316-1c4b-4281-b951-d872f2087c98