Remove ondemand mounting/unmounting of pools. Any operation that needs a
mounted pool just mounts it. The mount helper method actually mounts the pool
if it is not already mounted. This makes things simple and reduces error
scenarios.
Add low level code for scrubbing pools.
Create data model for pool scrubs. It should store meta data with inplace
updates during the life cycle of a scrub.
Implement API for pool scrubbing. Use POST to start a scrub or request status
of (the latest) scrub. Use GET for metadata of all scrubs historically.
Implement CLI support for pool scrub.
Merge branch 'master' into issue#175_pool_scrub

Conflicts:
	src/rockstor/storageadmin/models/__init__.py
	src/rockstor/storageadmin/serializers.py
	src/rockstor/storageadmin/views/__init__.py
Bugfix: Regenerate the conflicting db migration for pool scrub. Remove dead
code from prep_db
Make share unmount operation a necessary condition in remove_share. Provide a
mount suffix as an optional argument to check mounts.
Bugfix: umount nfs export mount when deleting the last export of a given share.
Don't unmount the share when sharing via samba is turned off even if there is
no active sharing/exporting going on for the share.
Don't allow share deletion when there are active snapshots, nfs exports or
samba shares for the given share. When these constraints are satisfied, clean
umount the share and only when successful, delete the share.
Merge branch 'issue#175_pool_scrub' of https://github.com/schakrava/rockstor-core into schakrava-issue#175_pool_scrub

Conflicts:
	src/rockstor/storageadmin/serializers.py