Provide control to initiate btrfs balance on btrfs-formatted array devices and single cache device.

Remove preset btrfs balance options; btrfs-raid1 is default only for initial creation of multi-device pool.
DeviceInfo shows all check/balance/scrub operations but greyed out depending on arry started state.

plugins/dynamix/scripts/btrfs_balance

@@ -5,7 +5,9 @@
 
 case "$1" in
 'start')
-  exec /sbin/btrfs balance start $3 $2 &>/dev/null &
+  options=$3
+  [[ -z "${options// }" ]] && options="--full-balance"
+  exec /sbin/btrfs balance start $options $2 &>/dev/null &
 ;;
 'status')
   /sbin/btrfs balance status $2