pool.sks-keyservers.net GPG key server is unreliable

The `pool.sks-keyservers.net` machines occasionally time-out trying to fetch
the GPG key, which leads to an acceptance test failure - because we can't get
the key required to install Puppet, which means we can't install Razor.

We should make sure we at least try more than once to fetch the key, and
ideally use a second source so that we increase the chance of successfully
finding the public key material.

This shouldn't change the overall robustness of the system because all the
various key servers exchange keying data with each other routinely, and we are
not looking for keys that rapidly change.

This implements that, trying with `pgp.mit.edu` and then `keyserver.pgp.com`
if we fail on fetching the key from sks; that should be fairly robust.

Closes #375.

Signed-off-by: Daniel Pittman <daniel@rimspace.net>
Merge pull request #376 from daniel-pittman/feature/master/375-increase-gpg-key-server-robustness

pool.sks-keyservers.net GPG key server is unreliable