fix(ssh): Only set HostKeyCallback if not nil

Fix #248
Merge pull request #249 from bcwaldon/fix-248

fix(ssh): Only set HostKeyCallback if not nil
test(functional): Add more logging
test(functional): s/fleet-local/fleet/
test(functional): Update for alpha filesystem
test(functional): robustify machine cleanup
test(functional): Add retry logic to machinectl call
test(functional): Robustify functional testing
test(functional): Expand README with etcd requirement
test(functional): factor out helper functions
Merge pull request #250 from bcwaldon/update-functional-tests

Update functional tests
test(functional): Fix false-positive
test(functional): Deal with empty return values properly
test(functional): Add test for known-hosts verification
Merge pull request #251 from bcwaldon/known-hosts-testing

Add functional test for known-hosts verification
bump(godbus/dbus): cb98efbb933d8389ab549a060e880ea3c375d213
bump(coreos/go-systemd): 4fbc5060a317b142e6c7bfbedb65596d5f0ab99b
chore(dep): Drop unused dep guelfey/go.dbus
Merge pull request #254 from bcwaldon/bump-go-systemd

Bump go-systemd; Use godbus/dbus
feat(agent): Allow to run jobs when the requirement in X-ConditionMachineBootID uses a short id.