RFE: allow systemd instances of maintenance

[jirib]

Now at

btrfsmaintenance/btrfs-balance.sh

Line 9 in be42cb6

 if [ -f /etc/sysconfig/btrfsmaintenance ] ; then 

we load /etc/sysconfig/btrfsmaintenance. This prevents having multiple instances of btrfsmaintenance, eg. per filesystems.

Could you consider improving that?