Use file-based storage for OAuth2 credentials if keyring is not available.
Use file-based storage for OAuth2 credentials if keyring is not available.

This fixes ImportError 'No module named keyring' when importing utils.oauth.

BUG=
R=maruel@chromium.org

Review URL: https://codereview.appspot.com/51760044

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py