[#6457] Make way for extending tmpl to hide mount fields

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
[#6457] Save result of mount_point validation

- Allows validator to return an alternate mount_point if the one
  passed in is already in use.

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>