Add delete methods to Group classes

Allow all members of the group to be deleted with one command.

Signed-off-by: Andy Grover <agrover@redhat.com>
Fix a for-loop variable name

'na' usually refers to a NodeACL, but in the Group class we don't know
what derived object we are calling setattr on, so make this clear with a
better loop variable name.
Use _get_first_member in NodeACLGroup.mapped_luns

Take more advantage of the base class's capabilities.

Signed-off-by: Andy Grover <agrover@redhat.com>
Add MappedLUNGroup.tpg_lun

This should allow easier linkage between Group classes and class LUN.

Signed-off-by: Andy Grover <agrover@redhat.com>