system-image fails to sign images if more than one key is available

We are in the process of rotating signing keys for system-image.ubuntu.com. Installing the new signing key results in a keyring containing only the new private key, but both old and new public keys.

This causes a backtrace in the importer:

2015-05-21 18:28:21,633 INFO Processing channel: ubuntu-touch/devel-proposed/ubuntu
2015-05-21 18:28:21,635 INFO Processing device: mako
2015-05-21 18:28:21,647 INFO Calling 'cdimage-ubuntu' generator for a new file
Traceback (most recent call last):
  File "/srv/system-image.ubuntu.com/bin/import-images", line 188, in <module>
    environment)
  File "/srv/system-image.ubuntu.com/bin/../lib/systemimage/generators.py", line 158, in generate_file
    path = generate_file_cdimage_ubuntu(conf, arguments, environment)
  File "/srv/system-image.ubuntu.com/bin/../lib/systemimage/generators.py", line 572, in generate_file_cdimage_ubuntu
    gpg.sign_file(conf, "image-signing", path)
  File "/srv/system-image.ubuntu.com/bin/../lib/systemimage/gpg.py", line 92, in sign_file
    [key] = ctx.keylist()
ValueError: too many values to unpack

This is a bug; the code should not be reading the list of *public* keys to determine which key to sign with, it should look at the available *private* keys.