Upgrade keyring installation

The Ubuntu 12 image on Linode has an old version of keyring installed (0.9.2) by default, which doesn't have `delete_password` like the latest version (4.0) does. We should upgrade that installation so the `unit.utils.cloud_test.CloudUtilsTestCase.test__save_password_in_keyring` test will pass on that VM.

@s0undt3ch - is this the correct way to do this, or is there a better option to use?