misc: Reimplement host key scanning

misc: Reimplement host key scanning

We're seeing very intermittent issues with ssh-keyscan; sometimes given
N hostnames, it only returns N-1 keys. Lack of an error message adds to
the confusion. The solution is to call ssh-keyscan once for each host
instead of batching them together - with a few retries - so that we can
easily ensure we get the right amount. If we do not, raise a
RuntimeError.

Signed-off-by: Zack Cerza <zack@redhat.com>
(cherry picked from commit daa28ae9210e1a845840ec80776bd211df2e97e9)