Import from ceph_deploy package instead of . in gatherkeys.py

Signed-off-by: Travis Rhoden <trhoden@redhat.com>
Return non-zero when gatherkeys fails

Immediately abort and exit when a needed key cannot be found
on the list of given hosts.

Define a new exception, KeyNotFoundError, and raise that from
gatherkeys() when the requested file does not exist on any of
the given hosts to try.  Since we now raise an exception on
error, there is no longer any need to return a value from
gatherkeys().

Fixes: 10043

Signed-off-by: Travis Rhoden <trhoden@redhat.com>
Merge pull request #250 from trhoden/10043

Return non-zero exit status when gatherkeys fails