fs: Fix template name for pivot-root directory

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fs: Mark fs in new NS as alave recursively

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
test: Add test for root-ed mntns container

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