Fix for clearing node MAINT node flag

Fix bug that was clearing MAINT mode on nodes scheduled for reboot (bug
    introduced in version 16.05.5 to address bug in overlapping reservations,
    commit 5eee1d28). Note that a node's
    MAINT flag is used for both a requested reboot and maintenance reservation.
    What I'd like to do is add a new node state flag to differenciate
    between these two cases, but that involves some significant changes
    that could introduce instability, so it will be defered to version
    17.02
bug 3210