check-tempest-dsvm-virtual-ironic fails if Ubuntu cloud archive GPG key is unavailable

[Adam Gandelman]

The ironic dsg job is unique in that it sets up access to the Ubuntu Cloud Archive. Occasionally, the GPG server that hosts the key for this apt archive returns NO_PUBKEY error, failing the job:

W: GPG error: http://ubuntu-cloud.archive.canonical.com precise-updates/icehouse Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5EDB1B62EC4926EA

This is a known transient issue and I do not know of a way to mitigate it in the short term other than installing the key into the images used by nodepool. In the long term, this will be resolved as we move slaves to Trusty and no longer require newer versions of things offered in UCA.