Add machine serial to repair queue entries

[chez-shanpu]

This PR adds machine serial number to repair queue.
Changes are below:

• Added Serial field to RepairQueueEntry struct
• Updated NewRepairQueueEntry() to accept serial parameter
• Modified CLI command signature: ckecli repair-queue add OPERATION MACHINE_TYPE ADDRESS [SERIAL]
• Updated Sabakan integration to pass machine serial when creating repair entries
• Updated all tests and documentation

The ckecli repair-queue add command now accepts an optional SERIAL parameter for backward compatibility.

# Before
$ ckecli repair-queue add unreachable type1 10.0.0.1
# New
$ ckecli repair-queue add unreachable type1 10.0.0.1 SN123456
# Also works
$ ckecli repair-queue add unreachable type1 10.0.0.1

[morimoto-cybozu]

This description seems misleading.
CKE does not use SERIAL to select a target machine.

[morimoto-cybozu]

This comment is not a request for change.
The following is purely a matter of preference.

Because we don't necessarily need SERIAL, we could make it an option.
ckecli repair-queue add OPERATION MACHINE_TYPE ADDRESS --serial SERIAL

We could make it an optional argument.
ckecli repair-queue add OPERATION MACHINE_TYPE ADDRESS [SERIAL]

Because we record SERIAL just for displaying target machine's informational details, we could put it in a comment option.
ckecli repair-queue add OPERATION MACHINE_TYPE ADDRESS --comment "Serial=DEADBEEF"

[chez-shanpu]

I see. Making it an option seems simple and good to me.
I'll fix it that way.

[chez-shanpu]

I made it an option and fixed PR's description.