Upgrade "scontrol reboot" logic

Add new node state flag of NODE_STATE_REBOOT for node reboots triggered by
    "scontrol reboot" commands. Previous logic re-used NODE_STATE_MAINT flag,
    which could lead to inconsistencies. Add "ASAP" option to "scontrol reboot"
    command that will drain a node in order to reboot it as soon as possible,
    then return it to service.
bug 3210