This implements `osctl cluster destroy` for Firecracker, adds
new utility command `osctl cluser show`.
Firecracker mode now has control process for firecracker VMs, allowing
clean reboots and background operations.
Lots of small fixes to Firecracker mode, clean CNI shutdown, cleaning up
netns, etc.
Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>
