Don't try to release 'NEXT'.

Since we stopped double-handling things, NEXT is present in NEWS when we do a
release.

Change-Id: I4f12c0da2856b5ec072328ad9185e060182c8791
Use a credentials file rather than global keyring.

Global keyring really doesn't work all that well in the places I tend to run
this script.

Change-Id: I3e926aed2a63ada0a23cb463582b50fea2bfc007