umount should lazy mount, wait up to 10 seconds and if the mount point is still
active, try force umount.
separate out snapshot removal from share removal for clarity.
Fixes #405

If a snapshot is supposed to be visible, create appropriate nfs export
configuration. when a snapshot is deleted, teardown if necessary.
Remove view error handling since global error handler is present
Display appropriate error message when user tries to delete a Share that has
replication turned on.
Delete multiple snapshots

Fix add nfs export cancel button
Implement multiple snapshot delete method
Add multi-snapshot delete capability to the API.

to delete multiple snapshots, the url should be something like:
/snapshots?id=2,3,4,5

The batch delete snapshots one by one and if an error occurs, it just returns
with the error message.
Merge branch 'issue#405' of github.com:schakrava/rockstor-core into issue#405
remove mount checking using os.path.ismount as bind mounts are not recognized
by it.
make the low level mount/umount helpers more robust.
Make the visibility of a snapshot independent from its Share's nfs export
configuration. This ensures that snapshots can be visible using SFTP or
Samba/CIFS even if the Share is not exported by nfs.
Update error message during snapshot deletion.
remove no_root_squash that was default before.
delete stub files in storageadmin
Delete multiple snapshots
Merge branch 'issue#405' of github.com:schakrava/rockstor-core into schakrava-issue#405
Merge pull request #419 from schakrava/issue#405

fixes issue#405