Fix root disk not be detached after deleting lxc container

Add teardown_container action when cleanup lxc instance to make
sure root disk be detached and no resource leak.

Fixes: bug #1208387
       bug #1203927

Change-Id: I063fd3a9856bba089bcde5cdefd2576e2eb2b0e9
Merge "Fix root disk not be detached after deleting lxc container"