zdtm.py: Add action for cleaning nsroot
zdtm.py: Add action for cleaning nsroot

When running test in ns/uns flavor zdtm.py builds a temp root in test/ directory.
The new action allows to remove all this stuff automatically.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

svloyso/criu,ldu4/criu,fbocharov/criu,fbocharov/criu,KKoukiou/criu-remote,eabatalov/criu,ldu4/criu,wtf42/criu,tych0/criu,svloyso/criu,fbocharov/criu,svloyso/criu,ldu4/criu,AuthenticEshkinKot/criu,ldu4/criu,wtf42/criu,AuthenticEshkinKot/criu,efiop/criu,efiop/criu,efiop/criu,tych0/criu,ldu4/criu,svloyso/criu,fbocharov/criu,eabatalov/criu,efiop/criu,AuthenticEshkinKot/criu,KKoukiou/criu-remote,wtf42/criu,KKoukiou/criu-remote,tych0/criu,wtf42/criu,wtf42/criu,fbocharov/criu,AuthenticEshkinKot/criu,wtf42/criu,svloyso/criu,KKoukiou/criu-remote,AuthenticEshkinKot/criu,efiop/criu,svloyso/criu,tych0/criu,eabatalov/criu,fbocharov/criu,KKoukiou/criu-remote,tych0/criu,tych0/criu,eabatalov/criu,ldu4/criu,eabatalov/criu,efiop/criu,AuthenticEshkinKot/criu,KKoukiou/criu-remote,eabatalov/criu