To support a safer take-over from the provisioning to the tenant network
for hardware that cannot be powered off, this change introduces a new
command system.lockdown. When invoked, it stops the API, the heartbeater
and disables all network interfaces (if possible).
Partial-Bug: #2077432
Change-Id: I211fc64a46226127b0d82ab458029b3c702b3f74
