Use stager minimum_task_memory from config rather than hardcoding
Don't stage on stagers without enough disk

Consider available_disk reported by stager before making staging
requests. This avoids errors which occur when cloud controller attempts
to stage on a DEA that's out of disk.
Update minimum_staging_disk_mb default to 4G.

[#69204488]

Signed-off-by: Kai Zhang <kaizhang@cn.ibm.com>