enable feature to automatically re-enable volumes

If we need to take volumes out of automatic management we'll have to set
disk_status to inactive.
sanity check before enabling volume

Most of the sanity checks are more applicable to new volumes but it
makes sense to always check before enabling a volume.
add use for required modules
also filter on can_allocate

It is necessary to also filter on can_allocate when mixing disable and
enable.
rewrite condition to isolate allowed threshold
increase threshold

This is mostly arbitrary but we want to disable volume that are unlikely
to be able to fit anymore data on them.  The more we scale this
threshold the more space we (temporarily) waste but the lesser we scale
this threshold the higher the chance of keeping unusable volumes in the
candidate volume pool.
Merge pull request #254 from nnutter/allocatable-cron

enable feature to automatically (re-)enable volumes