refactor(fleet/VerifyJob): abstract it as a function

Make it much clear.
fix(fleet/VerifyJob): check job from EventJobUpdated

When agent gets updated job from etcd, it should check its
signature still.
fix(agent): submit bids for jobs that agent can verify
refactor(fleetctl): Use an interface between cmd and registry to test commands on isolation.
Merge pull request #196 from unihorn/25

fix(fleet/VerifyJob): check job from EventJobUpdated
Merge pull request #198 from unihorn/27

chore(systemd): add some important debug info
Merge pull request #178 from calavera/cmd_bridge

refactor(fleetctl): Registry bridge interface.
feat(fleetctl): Use verify command instead of -verify flag
Merge pull request #197 from unihorn/26

feat(fleetctl): Use verify command instead of -verify flag
fix(agent/VerifyJob): cover all bid cases